avcodec/h264_cavlc: Set valid qscale value in ff_h264_decode_mb_cavlc()

When ff_h264_decode_mb_cavlc() failed due to wrong sl->qscale values,
e.g. dquant out of range, set the qscale to be a valid value before
returning -1 and exiting the function. The qscale value can be used
later e.g. in loop filter.

BUG=806122
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 71f39de2)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/h264_cavlc.c

@@ -1111,6 +1111,7 @@ decode_intra_mb:
             else                sl->qscale -= max_qp+1;
             if (((unsigned)sl->qscale) > max_qp){
                 av_log(h->avctx, AV_LOG_ERROR, "dquant out of range (%d) at %d %d\n", dquant, sl->mb_x, sl->mb_y);
+                sl->qscale = max_qp;
                 return -1;
             }
         }