Align internal mfqe framebuffer dimensions

MFQE postproc crashed with stream dimensions not a multiple of 16.
The buffer was memset unconditionally, so if the buffer allocation
fails we end up trying to write to NULL.

This patch traps an allocation failure with vpx_internal_error(),
and aligns the buffer dimensions to what vp8_yv12_alloc_frame_buffer()
expects.

Change-Id: I3915d597cd66886a24f4ef39752751ebe6425066