Fix [-Wimplicit-fallthrough] warnings in PDO extension

Author: Girgias

ext/pdo/pdo_stmt.c

@@ -1125,7 +1125,7 @@ static bool pdo_stmt_verify_mode(pdo_stmt_t *stmt, zend_long mode, uint32_t mode
 				zend_argument_value_error(mode_arg_num, "cannot be PDO::FETCH_LAZY in PDOStatement::fetchAll()");
 				return 0;
 			}
-			/* fall through */
+			fallthrough;
 		default:
 			if ((flags & PDO_FETCH_SERIALIZE) == PDO_FETCH_SERIALIZE) {
 				zend_argument_value_error(mode_arg_num, "must use PDO::FETCH_SERIALIZE with PDO::FETCH_CLASS");
@@ -1139,7 +1139,7 @@ static bool pdo_stmt_verify_mode(pdo_stmt_t *stmt, zend_long mode, uint32_t mode
 				zend_argument_value_error(mode_arg_num, "must be a bitmask of PDO::FETCH_* constants");
 				return 0;
 			}
-			/* no break; */
+			fallthrough;
 
 		case PDO_FETCH_CLASS:
 			return 1;