avcodec/snowenc: MV limits due to mv_penalty table size

Fixes: out of array read
Fixes: 69673/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_SNOW_fuzzer-5476592894148608

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 3a9292aff3)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/snowenc.c

@@ -411,6 +411,7 @@ static int encode_q_branch(SnowEncContext *enc, int level, int x, int y)
     int my_context= av_log2(2*FFABS(left->my - top->my));
     int s_context= 2*left->level + 2*top->level + tl->level + tr->level;
     int ref, best_ref, ref_score, ref_mx, ref_my;
+    int range = MAX_MV >> (1 + qpel);
 
     av_assert0(sizeof(s->block_state) >= 256);
     if(s->keyframe){
@@ -452,6 +453,11 @@ static int encode_q_branch(SnowEncContext *enc, int level, int x, int y)
     c->xmax = - (x+1)*block_w + (w<<(LOG2_MB_SIZE - s->block_max_depth)) + 16-3;
     c->ymax = - (y+1)*block_w + (h<<(LOG2_MB_SIZE - s->block_max_depth)) + 16-3;
 
+    c->xmin = FFMAX(c->xmin,-range);
+    c->xmax = FFMIN(c->xmax, range);
+    c->ymin = FFMAX(c->ymin,-range);
+    c->ymax = FFMIN(c->ymax, range);
+
     if(P_LEFT[0]     > (c->xmax<<shift)) P_LEFT[0]    = (c->xmax<<shift);
     if(P_LEFT[1]     > (c->ymax<<shift)) P_LEFT[1]    = (c->ymax<<shift);
     if(P_TOP[0]      > (c->xmax<<shift)) P_TOP[0]     = (c->xmax<<shift);