nutdec: fix type of intermediate variable for ts calculation.

Prior to this it could theoretically become negative and read
out of array.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/nutdec.c

@@ -527,7 +527,8 @@ static int decode_syncpoint(NUTContext *nut, int64_t *ts, int64_t *back_ptr)
 {
     AVFormatContext *s = nut->avf;
     AVIOContext *bc    = s->pb;
-    int64_t end, tmp;
+    int64_t end;
+    uint64_t tmp;
 
     nut->last_syncpoint_pos = avio_tell(bc) - 8;