PVS-Studio warning V575 for Fennec Media Project

int settings_proc_language_packs(....)
{
  ....
  case WM_DESTROY:
    if(mem_files)
    {
      mem_files = 0;
      sys_mem_free(mem_files);
    }
    EndDialog(hwnd,0);
    break;
  ....
}

This suspicious code was found in Fennec Media Project project by PVS-Studio static code analyzer.
Warning message is:
V575 The null pointer is passed into 'free' function. Inspect the first argument. settings interface.c 3096

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/