avformat/movenc: Fix undefined shift

Fixes the movenc FATE-test.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/movenc.c

@@ -4512,7 +4512,8 @@ static int mov_write_sidx_tag(AVIOContext *pb,
 {
     int64_t pos = avio_tell(pb), offset_pos, end_pos;
     int64_t presentation_time, duration, offset;
-    int starts_with_SAP, i, entries;
+    unsigned starts_with_SAP;
+    int i, entries;
 
     if (track->entry) {
         entries = 1;