lavf/asfdec: Reduce minimum header size.

Fixes GipsyGuitar.wmv mentioned in ticket #1477.

libavformat/asfdec.c

@@ -132,8 +132,9 @@ static const AVClass asf_class = {
 #include <assert.h>
 
 #define ASF_MAX_STREAMS 127
-#define FRAME_HEADER_SIZE 11
-// Fix Me! FRAME_HEADER_SIZE may be different. (17 is known to be too large)
+#define FRAME_HEADER_SIZE 6
+// Fix Me! FRAME_HEADER_SIZE may be different.
+// (7 is known to be too large for GipsyGuitar.wmv)
 
 #ifdef DEBUG
 static const ff_asf_guid stream_bitrate_guid = { /* (http://get.to/sdp) */