greybus: connection: protocol can be NULL in gb_connection_exit()

gb_connection_exit() is getting called from gb_connection_destroy() now,
which will get called from failure path of gb_connection_create_range()
(in a later commit). And at that point connection->protocol will be
NULL.

Don't print an error message if this happens in gb_connection_exit().

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>