Apply upstream fixes for a buffer overflow issue:
1571 Fix buffer overflow for forward reference within backward assertion
with excess closing parenthesis. Bugzilla 1651.

Obtained from: PCRE svn (r1571)
MFH: 2015Q3
Security: 8a1d0e63-1e07-11e5-b43d-002590263bf5
Security: CVE-2015-5073