avcodec/dnxhddec: Move mb height check out of non hr branch

      Fixes: out of array access
Fixes: poc.dnxhd

Found-by: Bingchang, Liu@VARAS of IIE
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 296debd213bd6dce7647cedd34eb64e5b94cdc92)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>