BUG/MINOR: arg: don't try to add an argument on failed memory allocation

Take care of arg_list_clone() returning NULL in arg_list_add() since
the former does it too. It's only used during parsing so the impact
is very low.

Can be backported to 1.7, 1.6 and 1.5.

src/arg.c

@@ -72,9 +72,11 @@ struct arg_list *arg_list_add(struct arg_list *orig, struct arg *arg, int pos)
 	struct arg_list *new;
 
 	new = arg_list_clone(orig);
+	if (new) {
 		new->arg = arg;
 		new->arg_pos = pos;
 		LIST_ADDQ(&orig->list, &new->list);
+	}
 	return new;
 }