PVS-Studio warning V503 for xpdfwin

char* stringarray_at(stringarray_t*sa, int pos)
{
  stringarray_internal_t*s =
    (stringarray_internal_t*)sa->internal;
  char *p;
  if (pos<0 || pos>=s->num)
    return 0;

  p = *(char**)&s->pos.buffer[pos*sizeof(char*)];

  if(p<0)  <<<---
    return 0;

  return p;
}

This suspicious code was found in xpdfwin project by PVS-Studio static code analyzer.
Warning message is:
V503 This is a nonsensical comparison: pointer < 0. pdf2swf q.c 804

PVS-Studio is a static analyzer for detecting bugs in the source code of applications written in C, C++, C++11, C++/CX. Site: http://www.viva64.com/en/pvs-studio/