MISRA Dereference of NULL pointer

I'm running a misra analysis with Perforce QAC tool on mine re-implementation of standard C library and facing this warnings

Apparent: de-reference of NULL pointer.
Suspicious: de-reference of NULL pointer.

Below my code:

char* strncat(char* destination, const char* source, size_t num)
{
    ASSERT(source != NULL);
    ASSERT(destination != NULL);
    ASSERT(num >= 0U);

    char* ptr = destination;

    /* Porta ptr alla termine di destination */
    while ((*destination != '
char* strncat(char* destination, const char* source, size_t num)
{
ASSERT(source != NULL);
ASSERT(destination != NULL);
ASSERT(num >= 0U);
char* ptr = destination;
/* Porta ptr alla termine di destination */
while ((*destination != '\0'))
{
*ptr++;
*destination++;
}
/* Aggiunge i caratteri di sorgente a destinazione */
while ((*source != '\0') && (num --))
{
*ptr = *source;
*ptr++;
*source++;
}
/* Aggiunge carattere di terminazione */
*ptr = '\0';
return destination;
}
'))
    {
        *ptr++;
        *destination++;
    }
    
    /* Aggiunge i caratteri di sorgente a destinazione */
    while ((*source != '
char* strncat(char* destination, const char* source, size_t num)
{
ASSERT(source != NULL);
ASSERT(destination != NULL);
ASSERT(num >= 0U);
char* ptr = destination;
/* Porta ptr alla termine di destination */
while ((*destination != '\0'))
{
*ptr++;
*destination++;
}
/* Aggiunge i caratteri di sorgente a destinazione */
while ((*source != '\0') && (num --))
{
*ptr = *source;
*ptr++;
*source++;
}
/* Aggiunge carattere di terminazione */
*ptr = '\0';
return destination;
}
') && (num --))
    {
        *ptr = *source;
        *ptr++;
        *source++;
    }
    /* Aggiunge carattere di terminazione */
    *ptr = '
char* strncat(char* destination, const char* source, size_t num)
{
ASSERT(source != NULL);
ASSERT(destination != NULL);
ASSERT(num >= 0U);
char* ptr = destination;
/* Porta ptr alla termine di destination */
while ((*destination != '\0'))
{
*ptr++;
*destination++;
}
/* Aggiunge i caratteri di sorgente a destinazione */
while ((*source != '\0') && (num --))
{
*ptr = *source;
*ptr++;
*source++;
}
/* Aggiunge carattere di terminazione */
*ptr = '\0';
return destination;
}
';
    
    return destination;
}

I got these warnings on while((*destination != '\0)) and on while ((*source != '\0') && (num --)). I thought that the assert is enough.

Can I ignore this warnings or should I fix them ? In which way ?

thanks.

Misc comments:

• Run-time asserts aren't really recommended for safety-related software. There's the issue about them being enabled or not, as well as the issue of them suddenly becoming "dead code" in production build. For defensive programming purposes, it is better practice to develop a proper run-time error handler which is always there and part of the program. Also standard C assert is implemented as "lie down to die" which is of course unacceptable in safety-related programs.
• Beyond unit testing and testing during development, those input sanitation checks don't belong inside this function but should be on the caller side. If you have any reason to believe that any of these pointers are null pointers or that the arguments are otherwise corrupt, the code for finding that belongs at the place where the data has a chance of taking such unexpected values.
• *ptr++; and *destination++; etc is nonsense statements, that's not how pointers work in C. You shouldn't be de-referencing the pointers, only increment them, this is basic stuff.
• Mixing ++/-- operators with other operators in the same expression is bad practice in general and also not MISRA compliant.
• There is no obvious de-reference of null pointers in this code. If there is one, it's inside your ASSERT. For the rest of the code, the warnings make no sense at all. You shouldn't ignore the warnings, you should find out what's causing them. The tool you are using could be broken, which is most often the case with MISRA checkers.
• Also, anyone speaking of "NULL pointers" may be mixing up terms (it's easy to do). There's null pointers, null pointer constants and the NULL macro. Related, but different terms.
• ((*destination != '\0')) These double parenthesis are just clutter and serve no purpose.
• As noted in comments, you shouldn't modify destination in case you intend to mimic standard C strncat.

Cleaned up code (not tested but should be MISRA compliant):

char* strncat(char* destination, const char* source, size_t num)
{
    char* end = destination;

    while(*end != '
char* strncat(char* destination, const char* source, size_t num)
{
char* end = destination;
while(*end != '\0')
{
end++;
}
for(size_t i=0; i<num; i++)  // keep loops as simple as possible
{
if(source[i] == '\0')
{
break;
}
*end = source[i];
end++;
}
*end = '\0';
return destination;
}
')
    {
      end++;
    }
        
    for(size_t i=0; i<num; i++)  // keep loops as simple as possible
    {
      if(source[i] == '
char* strncat(char* destination, const char* source, size_t num)
{
char* end = destination;
while(*end != '\0')
{
end++;
}
for(size_t i=0; i<num; i++)  // keep loops as simple as possible
{
if(source[i] == '\0')
{
break;
}
*end = source[i];
end++;
}
*end = '\0';
return destination;
}
')
      {
        break;
      }
      *end = source[i];
      end++;
    }
    *end = '
char* strncat(char* destination, const char* source, size_t num)
{
char* end = destination;
while(*end != '\0')
{
end++;
}
for(size_t i=0; i<num; i++)  // keep loops as simple as possible
{
if(source[i] == '\0')
{
break;
}
*end = source[i];
end++;
}
*end = '\0';
return destination;
}
';

    return destination;
}