QUARTZ: check for NULL before IReferenceClock_Release

[Alex Villací­s Lasso]

While trying to isolate an AVIFile bug, this other bug showed up. The 
function TransformFilter_Release tries to decrement a reference counter 
on a NULL pClock, which inmediately segfaults. This patch corrects this bug:

Changelog:
* Check for NULL before IReferenceClock_Release in TransformFilter_Release

Alex Villacís Lasso