mm-video: vidc: When freeing output buffer, clear pBuffer

pBuffer can potentially point to a stale OMX_BUFFERHEADERTYPE after
free_output_buffers is called. Normally, this doesn't cause a problem.
But on dynamic port reconfig, the stale buffer is assigned to pPrevBuff
in the double buffering mechanism and causes the stale output buffer to
be handed to the component.

Change-Id: Ic15df7be758509805aaf4827999fcb221ccbcf24
1 file changed