Description:
------------
I used the MySQLi object-oriented API to prepare, execute, and bind results for a query using the latest PHP 7 snapshot for Windows (x86 non thread safe). While testing, I noticed a subtle yet serious bug:

If one row has a non-null value for a field, and the next row has a null value for the same field, the bound variable will not be set to null when mysqli_stmt::fetch is called (it will retain its non-null value from the previous row).

For example, if I have a users table with the following rows:

username | age
-------- | ----
user1    | 25
user2    | NULL

Fetching the first row correctly binds "user1" and 25 to the variables passed to mysqli_stmt::bind_result, but after calling mysqli_stmt::fetch a second time, the variables contain "user2" and 25 (rather than "user2" and NULL).

Test script:
---------------
$mysqli = new mysqli("localhost", "user", "password", "database");
$stmt = $mysqli->prepare("SELECT username, age FROM users");
$stmt->execute();
$stmt->bind_result($username, $age);

$stmt->fetch();
var_dump([$username, $age]); // ["user1", 25]

$stmt->fetch();
var_dump([$username, $age]); // ["user2", 25]
// ^ bug - array should contain ["user2", NULL]

$stmt->close();