gh-113343: Fix error check on mmap(2) (#113342)

Fix error check on mmap(2)

It should check MAP_FAILED instead of NULL for error.

On mmap(2) man page:

  RETURN VALUE
       On success, mmap() returns a pointer to the mapped area.
       On error, the value MAP_FAILED (that is, (void *) -1) is
       returned, and errno is set to indicate the error.