nb_chapters should be unsigned

Originally committed as revision 13261 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/avformat.h

@@ -522,7 +522,7 @@ typedef struct AVFormatContext {
      */
     unsigned int max_picture_buffer;
 
-    int nb_chapters;
+    unsigned int nb_chapters;
     AVChapter **chapters;
 } AVFormatContext;