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>