Apply upstream patch r1498 to our PCRE copy

It's actually a subset of the patch (tests, docs, other whitespace
fixes were dropped).

Fixes a stack overflow issue on pathological regexps reported upstream:
http://bugs.exim.org/show_bug.cgi?id=1515



Change-Id: Ie36536e820d79ff842d90efa6bec22b701423793
Reviewed-by: Richard J. Moore <rich@kde.org>