Yield NULL from read_expression() on invalid input

lib/reader.c

@@ -123,6 +123,9 @@ const expression_t *
 read_expression(memory_pool_t *pool, const char *input, int len)
 {
 	expression_t *expression;
-	parse_expression(pool, input, len, &expression);
-	return expression;
+	const int used = parse_expression(pool, input, len, &expression);
+	if (used == len)
+		return expression;
+	else
+		return NULL;
 }