libs/ui: Fix crash issue because of null pointer access

- handle NULL check is required while flatten GraphicsBuffer

CRs-fixed: 579102

Change-Id: I292b4b25dc1f5320378cc9e4affbad326e1fdaed
SurfaceFlinger: Add NULL check for buffer handling

Add buffer handling NULL check in dequeueBuffer and
verify whether the output data from binder is not NULL
in queueBuffer and connect api's to avoid SF crash

CRs-Fixed: 573088
CRs-Fixed: 572315

Change-Id: I41cebbc0cbcbbb0fd5ecb38db7ec7b0c91cdffe9