avcodec/vc1_pred: Fix refdist in scaleforopp()

Fixes: out of array access
Fixes: 16601/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_VC1IMAGE_fuzzer-5656105392275456

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 413e0f2516)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/vc1_pred.c

@@ -197,9 +197,10 @@ static av_always_inline int scaleforopp(VC1Context *v, int n /* MV */,
         return n;
     }
     if (v->s.pict_type != AV_PICTURE_TYPE_B)
-        refdist = FFMIN(v->refdist, 3);
+        refdist = v->refdist;
     else
         refdist = dir ? v->brfd : v->frfd;
+    refdist = FFMIN(refdist, 3);
     scaleopp = ff_vc1_field_mvpred_scales[dir ^ v->second_field][0][refdist];
 
     n = (n * scaleopp >> 8) * (1 << hpel);