musicpal: Catch null TX qeueues

They likely represent invalid queues that should be skipped. We already
do this for RX queues. Wish I had a spec...

Credits go to malc for analyzing the issue and suggesting this fix.

Signed-off-by: Jan Kiszka <jan.kiszka@web.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
