Bug #2058: deny_info TCP_RESET crashes squid

Bug #2058: deny_info TCP_RESET crashes squid

There was a race condition in request processing, easily triggered by using
deny_info TCP_RESET and denying access. It's very likely the problem could
also be triggered by other conditions where Squid very quickly closes the
client connection.

Was caused by a malplaced isClosed() condition. Moved this down to after the
request processing where it belongs.

This patch also adds some safeguards make further request processing stop and
avoid risking referencing the fd as valid after close.

Additionally connStateFree was renamed to the more appropriate connStateClosed
as all it does is to make the connState aware that the underlying fd has been
closed.