nis: Fix leak on realloc failure in nis_getnames [BZ #28150]

If pos >= count but realloc fails, tmp will not have been placed in
getnames[pos] yet, and so will not be freed in free_null.  Detected
by Coverity.

Also remove misleading comment from nis_getnames(), since it actually
did properly release getnames when out of memory.

Tested-by: Carlos O'Donell <carlos@redhat.com>