avformat/wavdec: Use 64bit in new_pos computation

Fixes: signed integer overflow: 129 * 16711680 cannot be represented in type 'int'
Fixes: 29102/clusterfuzz-testcase-minimized-ffmpeg_dem_WAV_fuzzer-6742285317439488

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

libavformat/wavdec.c

@@ -718,7 +718,7 @@ smv_retry:
         if (wav->smv_last_stream) {
             uint64_t old_pos = avio_tell(s->pb);
             uint64_t new_pos = wav->smv_data_ofs +
-                wav->smv_block * wav->smv_block_size;
+                wav->smv_block * (int64_t)wav->smv_block_size;
             if (avio_seek(s->pb, new_pos, SEEK_SET) < 0) {
                 ret = AVERROR_EOF;
                 goto smv_out;