Fix stmt_free_result implementation and usage

Two bugs both affecting the bug_pecl_7976.phpt test ("works with
mysqlnd" haha):

 * We should not change the connection state in stmt_free_result.
   This makes mysql_stmt_free_result usable under mysqlnd and
   not just libmysqlclient.
 * If we call mysql_stmt_free_result, we still need to consume
   any outstanding result sets.

ext/mysqlnd/mysqlnd_ps.c

@@ -1931,10 +1931,6 @@ MYSQLND_METHOD(mysqlnd_stmt, free_result)(MYSQLND_STMT * const s)
 		stmt->state = MYSQLND_STMT_PREPARED;
 	}
 
-	if (GET_CONNECTION_STATE(&conn->state) != CONN_QUIT_SENT) {
-		SET_CONNECTION_STATE(&conn->state, CONN_READY);
-	}
-
 	DBG_RETURN(PASS);
 }
 /* }}} */

ext/pdo_mysql/mysql_statement.c

@@ -893,9 +893,7 @@ static int pdo_mysql_stmt_cursor_closer(pdo_stmt_t *stmt) /* {{{ */
 		S->result = NULL;
 	}
 	if (S->stmt) {
-		int retval;
-		retval = mysql_stmt_free_result(S->stmt);
-		PDO_DBG_RETURN(retval ? 0 : 1);
+		mysql_stmt_free_result(S->stmt);
 	}
 
 	while (mysql_more_results(S->H->server)) {

ext/pdo_mysql/tests/bug_pecl_7976.phpt

@@ -56,8 +56,6 @@ require __DIR__ . '/mysql_pdo_test.inc';
 $db = MySQLPDOTest::factory();
 $db->exec('DROP PROCEDURE IF EXISTS p');
 ?>
---XFAIL--
-Works with mysqlnd. It is not supported by libmysql. For libmysql is good enough to see no crash.
 --EXPECT--
 Emulated...
 array(1) {