avconv: only set the "channels" option when it exists for the specified input format

This allows the user to specify an input channel layout without avconv aborting because the
"channels" option was not found.

avconv.c

@@ -3627,8 +3627,16 @@ static int opt_input_file(OptionsContext *o, const char *opt, const char *filena
         av_dict_set(&format_opts, "sample_rate", buf, 0);
     }
     if (o->nb_audio_channels) {
-        snprintf(buf, sizeof(buf), "%d", o->audio_channels[o->nb_audio_channels - 1].u.i);
-        av_dict_set(&format_opts, "channels", buf, 0);
+        /* because we set audio_channels based on both the "ac" and
+         * "channel_layout" options, we need to check that the specified
+         * demuxer actually has the "channels" option before setting it */
+        if (file_iformat && file_iformat->priv_class &&
+            av_opt_find(&file_iformat->priv_class, "channels", NULL, 0,
+                        AV_OPT_SEARCH_FAKE_OBJ)) {
+            snprintf(buf, sizeof(buf), "%d",
+                     o->audio_channels[o->nb_audio_channels - 1].u.i);
+            av_dict_set(&format_opts, "channels", buf, 0);
+        }
     }
     if (o->nb_frame_rates) {
         av_dict_set(&format_opts, "framerate", o->frame_rates[o->nb_frame_rates - 1].u.str, 0);