hashtab: Add NULL check when destroying iterator.

The hashtab API is pretty NULL tolerant which has resulted
in remaining callers not doing much checks themselves.
Unfortunately the function to destroy an iterator does not
do a NULL check and will result in a crash if passed NULL.
This change fixes that.

ASTERISK-25552 #close

Change-Id: Ic1bf8eec3639e5a440f1c941d3ae3893ac6ed619

main/hashtab.c

@@ -746,6 +746,8 @@ struct ast_hashtab_iter *ast_hashtab_start_write_traversal(struct ast_hashtab *t
 
 void ast_hashtab_end_traversal(struct ast_hashtab_iter *it)
 {
+	if (!it)
+		return;
 	if (it->tab->do_locking)
 		ast_rwlock_unlock(&it->tab->lock);
 	free(it);