pts are unsigned according to specs, fix negative pts when 32bit pts are used

Originally committed as revision 12949 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/flvdec.c

@@ -280,7 +280,8 @@ static int flv_read_header(AVFormatContext *s,
 
 static int flv_read_packet(AVFormatContext *s, AVPacket *pkt)
 {
-    int ret, i, type, size, pts, flags, is_audio, next, pos;
+    int ret, i, type, size, flags, is_audio, next, pos;
+    unsigned pts;
     AVStream *st = NULL;
 
  for(;;){