lavfi/show_palette: fix memory leak

Recent commits 6aaac24d72 and
3835554bf8 made progress towards cleaning
up usage of the formats API, and in particular fixed possible NULL pointer
dereferences.

This commit addresses the issue of possible resource leaks when some intermediate
call fails. Unfortunately, even leaving aside this subtle intermediate
failure aspect, commit 8087632027 was only
partially successful in addressing memleaks. Hopefully, this commit
fixes the issue completely.

Tested with valgrind --leak-check=full --show-leak-kinds=all, and manual simulation
of malloc/realloc failures.

Fixes: CID 1270818.

Reviewed-by: Clément Bœsch <u@pkh.me>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

libavfilter/vf_showpalette.c

@@ -50,14 +50,22 @@ static int query_formats(AVFilterContext *ctx)
     AVFilterFormats *in  = ff_make_format_list(in_fmts);
     AVFilterFormats *out = ff_make_format_list(out_fmts);
     if (!in || !out) {
-        av_freep(&in);
-        av_freep(&out);
-        return AVERROR(ENOMEM);
+        ret = AVERROR(ENOMEM);
+        goto fail;
     }
+
     if ((ret = ff_formats_ref(in , &ctx->inputs[0]->out_formats)) < 0 ||
         (ret = ff_formats_ref(out, &ctx->outputs[0]->in_formats)) < 0)
-        return ret;
+        goto fail;
     return 0;
+fail:
+    if (in)
+        av_freep(&in->formats);
+    av_freep(&in);
+    if (out)
+        av_freep(&out->formats);
+    av_freep(&out);
+    return ret;
 }
 
 static int config_output(AVFilterLink *outlink)