PVS-Studio warning V579 for Firefox

int  AffixMgr::parse_convtable(..., const char * keyword)
{
  ...
  if (strncmp(piece, keyword, sizeof(keyword)) != 0) {
      HUNSPELL_WARNING(stderr,
        "error: line %d: table is corrupt\n", af->getlinenum());
      delete *rl;
      *rl = NULL;
      return 1;
  }
  ...
}

Most likely this is what should be written here: if (strncmp(piece, keyword, strlen(keyword)) != 0) {

This suspicious code was found in Firefox project by PVS-Studio static code analyzer.
Warning message is:
V579 The strncmp function receives the pointer and its size as arguments. It is possibly a mistake. Inspect the third argument. affixmgr.cpp 3708

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/