avcodec/vc1_block: Fix integer overflow in ff_vc1_pred_dc()

Fixes: signed integer overflow: 32796 * 65536 cannot be represented in type 'int'
Fixes: 15430/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_VC1IMAGE_fuzzer-5735424087031808

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

libavcodec/vc1_block.c

@@ -430,12 +430,12 @@ static inline int ff_vc1_pred_dc(MpegEncContext *s, int overlap, int pq, int n,
     if (c_avail && (n != 1 && n != 3)) {
         q2 = FFABS(s->current_picture.qscale_table[mb_pos - 1]);
         if (q2 && q2 != q1)
-            c = (c * s->y_dc_scale_table[q2] * ff_vc1_dqscale[dqscale_index] + 0x20000) >> 18;
+            c = (int)((unsigned)c * s->y_dc_scale_table[q2] * ff_vc1_dqscale[dqscale_index] + 0x20000) >> 18;
     }
     if (a_avail && (n != 2 && n != 3)) {
         q2 = FFABS(s->current_picture.qscale_table[mb_pos - s->mb_stride]);
         if (q2 && q2 != q1)
-            a = (a * s->y_dc_scale_table[q2] * ff_vc1_dqscale[dqscale_index] + 0x20000) >> 18;
+            a = (int)((unsigned)a * s->y_dc_scale_table[q2] * ff_vc1_dqscale[dqscale_index] + 0x20000) >> 18;
     }
     if (a_avail && c_avail && (n != 3)) {
         int off = mb_pos;
@@ -445,7 +445,7 @@ static inline int ff_vc1_pred_dc(MpegEncContext *s, int overlap, int pq, int n,
             off -= s->mb_stride;
         q2 = FFABS(s->current_picture.qscale_table[off]);
         if (q2 && q2 != q1)
-            b = (b * s->y_dc_scale_table[q2] * ff_vc1_dqscale[dqscale_index] + 0x20000) >> 18;
+            b = (int)((unsigned)b * s->y_dc_scale_table[q2] * ff_vc1_dqscale[dqscale_index] + 0x20000) >> 18;
     }
 
     if (c_avail && (!a_avail || abs(a - b) <= abs(b - c))) {