Fix [-Wimplicit-fallthrough] warnings in PDO extension

Author: Girgias

ext/pdo/pdo_stmt.c

@@ -554,6 +554,7 @@ static inline void fetch_value(pdo_stmt_t *stmt, zval *dest, int colno, int *typ
 				ZVAL_STRINGL(dest, value, value_len);
 				break;
 			}
+			fallthrough;
 		default:
 			ZVAL_NULL(dest);
 	}
@@ -1155,7 +1156,7 @@ 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");
@@ -1169,7 +1170,7 @@ 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;