Quick fix to stop vpxdec infinite loop

vpxdec stays in a loop when decoding vp9.  This patch is a
quick fix to stop the loop when all data has been decoded.
Eventually we should move the vp9_get_raw_frame() call into
vp8_get_frame().

Change-Id: I3f97b6b4e1fe9ba69f746a29fc54c3304956f661