* hashtab.c (htab_expand): Change to return int. Use calloc or

	* hashtab.c (htab_expand): Change to return int.  Use calloc or
 	xcalloc depending on htab->return_allocation_failure.  Return zero
 	if calloc fails.
	(htab_create): Update comment to cover memory allocation.
	(htab_try_create): New.
	(htab_find_slot_with_hash): Return NULL if htab_expand fails.
  	Update comment to cover this.