Fix crashes in btif_sendmsg when worqueue not initialized

Instead of crashing whole stack when trying to post task to
uninitialized queue, log error message. This can happen in two cases:
stack is not initialized yet, or we're shutting down and already cleaned
up the workqueue thread.

Bug: 28889608
Change-Id: I9b8652d27abc2489b2489997c928634b68433a3e