mov_probe: fix integer overflows

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mov.c

@@ -2858,7 +2858,7 @@ static int mov_read_default(MOVContext *c, AVIOContext *pb, MOVAtom atom)
 
 static int mov_probe(AVProbeData *p)
 {
-    unsigned int offset;
+    int64_t offset;
     uint32_t tag;
     int score = 0;