avcodec/vc1_block: Fix invalid shifts in vc1_decode_i_blocks()

Fixes: left shift of negative value -9
Fixes: 15299/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MSS2_fuzzer-5660922678345728
Fixes: 15557/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_VC1IMAGE_fuzzer-5673351911047168

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

@@ -2602,13 +2602,13 @@ static void vc1_decode_i_blocks(VC1Context *v)
                 if (v->rangeredfrm)
                     for (k = 0; k < 6; k++)
                         for (j = 0; j < 64; j++)
-                            v->block[v->cur_blk_idx][block_map[k]][j] <<= 1;
+                            v->block[v->cur_blk_idx][block_map[k]][j] *= 2;
                 vc1_put_blocks_clamped(v, 1);
             } else {
                 if (v->rangeredfrm)
                     for (k = 0; k < 6; k++)
                         for (j = 0; j < 64; j++)
-                            v->block[v->cur_blk_idx][block_map[k]][j] = (v->block[v->cur_blk_idx][block_map[k]][j] - 64) << 1;
+                            v->block[v->cur_blk_idx][block_map[k]][j] = (v->block[v->cur_blk_idx][block_map[k]][j] - 64) * 2;
                 vc1_put_blocks_clamped(v, 0);
             }