avcodec/vc1dec: Limit bits by the actual bitstream size

Fixes: Timeout (350 ->19sec)
Fixes: 19249/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_VC1IMAGE_fuzzer-6566896438870016

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/vc1dec.c

@@ -1033,7 +1033,7 @@ static int vc1_decode_frame(AVCodecContext *avctx, void *data,
 
         ff_mpeg_er_frame_start(s);
 
-        v->bits = buf_size * 8;
+        v->bits = FFMIN(buf_size * 8, s->gb.size_in_bits);
         v->end_mb_x = s->mb_width;
         if (v->field_mode) {
             s->current_picture.f->linesize[0] <<= 1;
@@ -1107,8 +1107,10 @@ static int vc1_decode_frame(AVCodecContext *avctx, void *data,
                 continue;
             }
             ff_vc1_decode_blocks(v);
-            if (i != n_slices)
+            if (i != n_slices) {
                 s->gb = slices[i].gb;
+                v->bits = FFMIN(buf_size * 8, s->gb.size_in_bits);
+            }
         }
         if (v->field_mode) {
             v->second_field = 0;