surfaceflinger_client: Don't dereference NULL ptrs.
surfaceflinger_client: Don't dereference NULL ptrs.

This change adds a null pointer check before dereferencing buffers in
a Surface list of known GraphicBuffer objects.  If not all the buffers
have been dequeued before this list can legitimately contain NULL
entries.

Change-Id: I0e105b53a3f6f9a5404716b7bf150c793ff648a6

xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base