fireworks/bebob: Improve comments for period/buffer boundary
firewire-lib: Fix error pointer reference just after stopping slave stream

Even is slave stream is stopped and an isochronous context for slave is
destroyed, master stream is still running. Then an isochronous context for
master still has interrupt callback. This causes error pointer reference at
queueing isochronous packets for slave.

This commit prevents from this case by checking isochronous context at
queue_packet().