avformat/nutenc: Fix writing strings larger than 2gb

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

libavformat/nutenc.c

@@ -280,7 +280,7 @@ static void put_tt(NUTContext *nut, AVRational *time_base, AVIOContext *bc, uint
  */
 static void put_str(AVIOContext *bc, const char *string)
 {
-    int len = strlen(string);
+    size_t len = strlen(string);
 
     ff_put_v(bc, len);
     avio_write(bc, string, len);