avfilter/af_channelsplit: Assert that av_channel_layout_channel_from_index() succeeds

Maybe Helps: CID1503077 Bad bit shift operation

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit cef720ab42)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/af_channelsplit.c

@@ -22,7 +22,7 @@
  *
  * Split an audio stream into per-channel streams.
  */
-
+#include "libavutil/avassert.h"
 #include "libavutil/attributes.h"
 #include "libavutil/channel_layout.h"
 #include "libavutil/internal.h"
@@ -162,6 +162,8 @@ static int filter_frame(AVFilterLink *outlink, AVFrame *buf)
     enum AVChannel channel = av_channel_layout_channel_from_index(&buf->ch_layout, s->map[i]);
     int ret;
 
+    av_assert1(channel >= 0);
+
     AVFrame *buf_out = av_frame_clone(buf);
     if (!buf_out)
         return AVERROR(ENOMEM);