avcodec/wmalosslessdec: Fix integer overflow with sliding in padding bits

Fixes: signed integer overflow: -53716100 * 256 cannot be represented in type 'int'
Fixes: 20143/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WMALOSSLESS_fuzzer-5716604000403456

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

libavcodec/wmalosslessdec.c

@@ -992,7 +992,7 @@ static int decode_subframe(WmallDecodeCtx *s)
             if (s->bits_per_sample == 16) {
                 *s->samples_16[c]++ = (int16_t) s->channel_residues[c][j] * (1 << padding_zeroes);
             } else {
-                *s->samples_32[c]++ = s->channel_residues[c][j] * (256 << padding_zeroes);
+                *s->samples_32[c]++ = s->channel_residues[c][j] * (256U << padding_zeroes);
             }
         }
     }