tools/firewire: nosy-dump: work around segfault in decode_fcp

If I run "nosy-dump --view=transaction" with my camcorder on battery
instead of mains, it segfaults very quickly because of !t->request.
Perhaps this is because of increased likelyhood of incomplete
transactions (ack_busy when host writes to camcorder's FCP_Request)
and a bug deeper in nosy-dump's transaction housekeeping.  This is a
quick workaround to get me going.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>