Description:
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Reading mysqli_stmt::num_rows property or calling the method mysqli_stmt::num_rows() should throw an error if using unbuffered results and the result hasn't been fully fetched yet. 

Test script:
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<?php

mysqli_report(MYSQLI_REPORT_ERROR | MYSQLI_REPORT_STRICT);
$mysqli = new mysqli('localhost', 'user', 'password', 'test');
$mysqli->set_charset('utf8mb4'); // always set the charset

$stmt = $mysqli->prepare('SELECT * FROM a LIMIT 2');
$stmt->execute();
echo $stmt->num_rows; // <-- Should trigger OOS error

Expected result:
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Out of sync error

Actual result:
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0