ffmpeg_opt: Check ist is valid before using it in audio_channels_map init

Fixes segfault and instead provides an error message
Fixes Ticket3465
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg_opt.c

@@ -1430,10 +1430,21 @@ static OutputStream *new_audio_stream(OptionsContext *o, AVFormatContext *oc, in
         /* check for channel mapping for this audio stream */
         for (n = 0; n < o->nb_audio_channel_maps; n++) {
             AudioChannelMap *map = &o->audio_channel_maps[n];
-            InputStream *ist = input_streams[ost->source_index];
-            if ((map->channel_idx == -1 || (ist->file_index == map->file_idx && ist->st->index == map->stream_idx)) &&
-                (map->ofile_idx   == -1 || ost->file_index == map->ofile_idx) &&
+            if ((map->ofile_idx   == -1 || ost->file_index == map->ofile_idx) &&
                 (map->ostream_idx == -1 || ost->st->index  == map->ostream_idx)) {
+                InputStream *ist;
+
+                if (map->channel_idx == -1) {
+                    ist = NULL;
+                } else if (ost->source_index < 0) {
+                    av_log(NULL, AV_LOG_FATAL, "Cannot determine input stream for channel mapping %d.%d\n",
+                           ost->file_index, ost->st->index);
+                    continue;
+                } else {
+                    ist = input_streams[ost->source_index];
+                }
+
+                if (!ist || (ist->file_index == map->file_idx && ist->st->index == map->stream_idx)) {
                     if (ost->audio_channels_mapped < FF_ARRAY_ELEMS(ost->audio_channels_map))
                         ost->audio_channels_map[ost->audio_channels_mapped++] = map->channel_idx;
                     else
@@ -1442,6 +1453,7 @@ static OutputStream *new_audio_stream(OptionsContext *o, AVFormatContext *oc, in
                 }
             }
         }
+    }
 
     if (ost->stream_copy)
         check_streamcopy_filters(o, oc, ost, AVMEDIA_TYPE_AUDIO);