avcodec/vc1_block: Fix integer overflow in ac value

Fixes: signed integer overflow: 25488 * 87381 cannot be represented in type 'int'
Fixes: 24765/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_VC1_fuzzer-5108259565076480

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

@@ -1080,7 +1080,7 @@ static int vc1_decode_intra_block(VC1Context *v, int16_t block[64], int n,
                     q2 = FFABS(q2) * 2 + ((q2 < 0) ? 0 : v->halfpq) - 1;
                 if (q2 && q1 != q2) {
                     for (k = 1; k < 8; k++)
-                        ac_val2[k] = (ac_val2[k] * q2 * ff_vc1_dqscale[q1 - 1] + 0x20000) >> 18;
+                        ac_val2[k] = (int)(ac_val2[k] * (unsigned)q2 * ff_vc1_dqscale[q1 - 1] + 0x20000) >> 18;
                 }
             }
         } else { // top
@@ -1093,7 +1093,7 @@ static int vc1_decode_intra_block(VC1Context *v, int16_t block[64], int n,
                     q2 = FFABS(q2) * 2 + ((q2 < 0) ? 0 : v->halfpq) - 1;
                 if (q2 && q1 != q2) {
                     for (k = 1; k < 8; k++)
-                        ac_val2[k + 8] = (ac_val2[k + 8] * q2 * ff_vc1_dqscale[q1 - 1] + 0x20000) >> 18;
+                        ac_val2[k + 8] = (int)(ac_val2[k + 8] * (unsigned)q2 * ff_vc1_dqscale[q1 - 1] + 0x20000) >> 18;
                 }
             }
         }