fix [4093196] Device lock up - log spam with SharedBufferStack:...

fix [4093196] Device lock up - log spam with SharedBufferStack: waitForCondition(LockCondition) timed out

a memory corruption happned when the buffer pool was resized
(like when playing a video or using camera) and there was
no current active buffer. In this case, the faulty code
would index into an array at position -1 which corrupted
24 bytes of data.

also improved region validation code (ifdef'ed out by default)

Bug: 4093196
Change-Id: I915c581d131148959d720e00e3892e9186ab733d