Out-of-bounds reads in zif_grapheme_stripos with negative offset

How it was found

https://bugs.php.net/bug.php?id=72061

grapheme_stripos from the intl extension had a security issue when handling negative offsets, this allowed to read from arbitrary memory locations.

Reported to developers on 2016-04-24, fixed 2016-04-29 and released at 2016-04-28, affected PHP 5.5 , 5.6 and 7.

http://php.net/ChangeLog-5.php#5.5.35 http://php.net/ChangeLog-5.php#5.6.21 http://php.net/ChangeLog-7.php#7.0.6