ipmi:ssif: Check for NULL msg when handling events and messages

Even though it's not possible to get into the SSIF_GETTING_MESSAGES and
SSIF_GETTING_EVENTS states without a valid message in the msg field,
it's probably best to be defensive here and check and print a log, since
that means something else went wrong.

Also add a default clause to that switch statement to release the lock
and print a log, in case the state variable gets messed up somehow.

Reported-by: Haowen Bai <baihaowen@meizu.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>