replace assert with check in ExecutorPool

This fix is needed to prevent make simple-test from failing
due to MB-11287 where an assert fires even when dead tasks
that have no taskType call doneTask()

Change-Id: I0b1ced312b40a2cce8e6b4b5d7ad5f6536292273
Reviewed-on: http://review.couchbase.org/38148
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: Sundararaman Sridharan <sundar@couchbase.com>