avformat/wtvdec: Avoid (32bit signed) sectors

Fixes: left shift of negative value -14614752
Fixes: 15174/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5670543606415360

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

libavformat/wtvdec.c

@@ -149,7 +149,7 @@ static int read_ints(AVIOContext *pb, uint32_t *data, int count)
  * @param depth         File allocation table depth
  * @return NULL on error
  */
-static AVIOContext * wtvfile_open_sector(int first_sector, uint64_t length, int depth, AVFormatContext *s)
+static AVIOContext * wtvfile_open_sector(unsigned first_sector, uint64_t length, int depth, AVFormatContext *s)
 {
     AVIOContext *pb;
     WtvFile *wf;
@@ -957,7 +957,8 @@ static int parse_chunks(AVFormatContext *s, int mode, int64_t seekts, int *len_p
 static int read_header(AVFormatContext *s)
 {
     WtvContext *wtv = s->priv_data;
-    int root_sector, root_size;
+    unsigned root_sector;
+    int root_size;
     uint8_t root[WTV_SECTOR_SIZE];
     AVIOContext *pb;
     int64_t timeline_pos;