Fix bogus fallthrough path in firebird_handle_get_attribute(), again

This reapplies b8e9c5ba6a after it was accidentally removed via
225034dbbc.

ext/pdo_firebird/firebird_driver.c

@@ -1250,8 +1250,7 @@ static int pdo_firebird_get_attribute(pdo_dbh_t *dbh, zend_long attr, zval *val)
 				ZVAL_STRING(val, tmp);
 				return 1;
 			}
-			/* TODO Check this is correct? */
-			ZEND_FALLTHROUGH;
+			return -1;
 
 		case PDO_ATTR_FETCH_TABLE_NAMES:
 			ZVAL_BOOL(val, H->fetch_table_names);