squid: Apply fix for Squid Advisory SQUID-2015:2

squid: Apply fix for Squid Advisory SQUID-2015:2

Squid configured with cache_peer and operating on explicit proxy
traffic does not correctly handle CONNECT method peer responses.

The bug is important because it allows remote clients to bypass
security in an explicit gateway proxy.

However, the bug is exploitable only if you have configured
cache_peer to receive CONNECT requests.

  http://www.squid-cache.org/Advisories/SQUID-2015_2.txt

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>