Fix implicit cast warning

Patch by Eli Friedman, eli dot friedman at gmail

Originally committed as revision 23817 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/rtpdec_asf.c

@@ -143,7 +143,7 @@ static int asfrtp_parse_sdp_line(AVFormatContext *s, int stream_index,
 
 struct PayloadContext {
     ByteIOContext *pktbuf, pb;
-    char *buf;
+    uint8_t *buf;
 };
 
 /**