avfilter/af_channelmap: fix error message if FL source channel was missing

FL channel ID is 0, so for an unset value we must check for ID < 0.

Regression since 1f96db959c.

Signed-off-by: Marton Balint <cus@passwd.hu>

libavfilter/af_channelmap.c

@@ -165,7 +165,7 @@ static av_cold int channelmap_init(AVFilterContext *ctx)
 
     for (i = 0; i < map_entries; i++) {
         int in_ch_idx = -1, out_ch_idx = -1;
-        int in_ch = 0, out_ch = 0;
+        int in_ch = -1, out_ch = -1;
         static const char err[] = "Failed to parse channel map\n";
         switch (mode) {
         case MAP_ONE_INT:
@@ -349,7 +349,7 @@ static int channelmap_config_input(AVFilterLink *inlink)
 
         if (m->in_channel_idx < 0 || m->in_channel_idx >= nb_channels) {
             av_channel_layout_describe(&inlink->ch_layout, layout_name, sizeof(layout_name));
-            if (m->in_channel) {
+            if (m->in_channel >= 0) {
                 av_channel_name(channel_name, sizeof(channel_name), m->in_channel);
                 av_log(ctx, AV_LOG_ERROR,
                        "input channel '%s' not available from input layout '%s'\n",