af_channelmap: properly set the supported output channel layouts

The current code expects query_formats() to be called exactly once, it
will leak if it's not called at all (filter initialized, but never
configured or used) or try to read freed memory if it's called more than
once.
Found-by: James Almer <jamrial@gmail.com>
CC: libav-stable@libav.org

libavfilter/af_channelmap.c

@@ -57,7 +57,6 @@ enum MappingMode {
 #define MAX_CH 64
 typedef struct ChannelMapContext {
     const AVClass *class;
-    AVFilterChannelLayouts *channel_layouts;
     char *mapping_str;
     char *channel_layout_str;
     uint64_t output_layout;
@@ -276,8 +275,6 @@ static av_cold int channelmap_init(AVFilterContext *ctx)
         return AVERROR(EINVAL);
     }
 
-    ff_add_channel_layout(&s->channel_layouts, s->output_layout);
-
     if (mode == MAP_PAIR_INT_STR || mode == MAP_PAIR_STR_STR) {
         for (i = 0; i < s->nch; i++) {
             s->map[i].out_channel_idx = av_get_channel_layout_channel_index(
@@ -291,11 +288,14 @@ static av_cold int channelmap_init(AVFilterContext *ctx)
 static int channelmap_query_formats(AVFilterContext *ctx)
 {
     ChannelMapContext *s = ctx->priv;
+    AVFilterChannelLayouts *channel_layouts = NULL;
+
+    ff_add_channel_layout(&channel_layouts, s->output_layout);
 
     ff_set_common_formats(ctx, ff_planar_sample_fmts());
     ff_set_common_samplerates(ctx, ff_all_samplerates());
     ff_channel_layouts_ref(ff_all_channel_layouts(), &ctx->inputs[0]->out_channel_layouts);
-    ff_channel_layouts_ref(s->channel_layouts,       &ctx->outputs[0]->in_channel_layouts);
+    ff_channel_layouts_ref(channel_layouts,          &ctx->outputs[0]->in_channel_layouts);
 
     return 0;
 }