Description:
------------
Processing bad utf-16 strings leads to unexpected source data changes. 

Test script:
---------------
<?php
$bad_utf16be = pack("H*", "dc00dc00"); /* two 1 byte chars */
$good_utf16be = pack("H*", "d800dc00"); /* one 2 bytes char */
$utf16_string = $bad_utf16be . $good_utf16be;

/* take second and third char  */
$second = mb_substr($utf16_string, 1, 1, "UTF-16BE");
$third = mb_substr($utf16_string, 2, 1, "UTF-16BE");
echo "\nactual:\n";
echo "second UTF-16BE char: 0x". bin2hex($second) . PHP_EOL;
echo "third  UTF-16BE char: 0x". bin2hex($third) . PHP_EOL;

echo "\nexpected:\n";
echo "second UTF-16BE char: 0xdc00" . PHP_EOL;
echo "third  UTF-16BE char: 0xd800dc00" . PHP_EOL;



Expected result:
----------------
expected:
second UTF-16BE char: 0xdc00
third  UTF-16BE char: 0xd800dc00

Actual result:
--------------
actual:
second UTF-16BE char: 0x003f
third  UTF-16BE char: 0xd800dc00