Squid-HEAD 0 byte read fix

Thanks to Vladimir B. Savkin for identifying a 0 byte read bug in the
processing of client requests, causing CPU spikes on requests with large
request entities (such as a large POST/PUT).

This bug has now been fixed in HEAD, and I wonder if this bug could be
the source of similar issues discussed some time ago here on squid-dev.

Note: This bug is specific HEAD/auth_rewrite/ntlm, introduced by the
changes in request processing I did as part of the NTLM integration.

--
Henrik