The bug has been fixed in an official release
A patch has been produced for daily versions but not yet distributed for regular builds, according to researchers.
It's Bug 4501, fixed in 3.5.18
http://bazaar.launchpad.net/~squid/squid/3.5/view/head:/ChangeLog
Changes to squid-3.5.19 (09 May 2016):
- Regression Bug 4515: interception proxy hangs
Changes to squid-3.5.18 (06 May 2016):
- Bug 4510: stale comment about 32KB limit on shared memory cache entries
- Bug 4509: EUI compile error on NetBSD
- Bug 4501: HTTP/1.1: normalize Host header
- Bug 4498: URL-unescape the login-info after extraction from URI
- Bug 4455: SegFault from ESIInclude::Start
- Prevent Squid forcing -b 2048 into the arguments for sslcrtd_program
- Fix TLS/SSL server handshake alert handling