cavsdec: unref frame before referencing again

This fixes asserts (from commit 13aae8) in av_frame_ref and
av_frame_move_ref.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavcodec/cavsdec.c

@@ -1217,6 +1217,8 @@ static int cavs_decode_frame(AVCodecContext *avctx, void *data, int *got_frame,
                 h->got_keyframe = 1;
             }
         case PIC_PB_START_CODE:
+            if (*got_frame)
+                av_frame_unref(data);
             *got_frame = 0;
             if (!h->got_keyframe)
                 break;