Add missing assert that task isn't queued

Probably won't fix anything but this assert is in queue()
but not queueAtFront() and inserting a task twice is Really Bad.

Bug: 36139852
Test:  ¯\_(ツ)_/¯
Change-Id: Ida0f829eecfdd46c17c36b816528c49d12b7cf29
1 file changed