Description:
------------
Prepare does not clean the wire before the next prepare. It should be discussed whether it should be done but this bug is created to have a track of this issue.

Reproduce code:
---------------
<?php

$st = $db->prepare("SELECT 1 AS test");
$st->execute();
$st->bind_result($x);
$st->fetch();
var_dump($x);

$st = $db->prepare("SELECT 1.23 AS test");
var_dump($st);
$st->execute();
$st->bind_result($x);
$st->fetch();
var_dump($x);

?>

Expected result:
----------------
int(1)
object(mysqli_stmt)#3 (0) {
}
string(4) "1.23"


Actual result:
--------------
int(1)
bool(false)
PHP Fatal error:  Call to a member function execute() on a non-object in /home/andrey/tst/ttt.php on line 11