Decrement size of pending write queue by correct amount

The pending write events vector is destructed before using its size to
decrement the number of events on the pending write events queue which
may cause the value decrement by 0 instead of the actual number of
events removed.

Bug: 147686560
Test: Build and load onto device with fake subhals
Change-Id: I7b6c6231eacfa89b0dd55bc405b235d34be5ff46