fix null protocol dereference when closing session

fix null protocol dereference when closing session

Thanks to Christopher Baker for pointing out that when we close a session,
if the close is coming before a protocol was negotiated for the connection
or when the protocol was otherwise left at NULL, we'll blow a segfault.

This implements his proposed fix.

Reported-by: Christopher Baker <me@christopherbaker.net>
Signed-off-by: Andy Green <andy@warmcat.com>