In the sliver of time between choosing a waiting job to be executed and
trying to acquire locks for its execution, the status of the job can be
changed to canceling. An assert checking the job status neglected to
take this into account, and raised an error that managed to perpetually
lock the job in the canceling state. This patch resolves the issue by
making the assert accept the canceling state as well, and exiting if
the job was cancelled.

Signed-off-by: Hrvoje Ribicic <riba@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>