Don't crash in WaitForAllTasks() if no tasks were ever enqueued.
Don't crash in WaitForAllTasks() if no tasks were ever enqueued.

patwonder/pbrt-v2-skin,patwonder/pbrt-v2-skin,patwonder/pbrt-v2-skin,patwonder/pbrt-v2-skin,patwonder/pbrt-v2-skin