nb_chapters should be unsigned

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@13261 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

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;