lavc: limit maximum number of channels to 63

This is the most that can be represented with the current channel layout
system. This limit should be raised/removed when a better system is
implemented.

libavcodec/internal.h

@@ -31,7 +31,7 @@
 #include "libavutil/pixfmt.h"
 #include "avcodec.h"
 
-#define FF_SANE_NB_CHANNELS 128U
+#define FF_SANE_NB_CHANNELS 63U
 
 typedef struct FramePool {
     /**