If the server goes away while the client is trying to copy a message from

If the server goes away while the client is trying to copy a message from
it, the kernel may panic with a null-pointer indirection through the caps
ci_td field (which becomes NULL).  The field is properly becoming NULL,
the culprit was some debugging code that indirected through it without
checking first.

Reported-by: Chris Pressey <cpressey@catseye.mine.nu>