Replace assert with error message

The failure happens when vendor code sends EMPTY_BUFFER_DONE
twice on the same buffer.  The second time through, we see a slot
that has already been emptied, and get very confused.

This should be handled with an error, not an assertion, so we now
log a stern complaint and carry on.

Bug 11557463

Change-Id: I76e53ff08924bca81afb6980df8e59a6012ff8fe