lavfi/vf_alphamerge: fix memory leaks

Recent commits 6aaac24d and
3835554b 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.

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

Fixes: CID 1338326, 1338329.
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

libavfilter/vf_alphamerge.c

@@ -61,13 +61,23 @@ static int query_formats(AVFilterContext *ctx)
     int ret;
 
     if (!(main_formats = ff_make_format_list(main_fmts)) ||
-        !(alpha_formats = ff_make_format_list(alpha_fmts)))
-        return AVERROR(ENOMEM);
+        !(alpha_formats = ff_make_format_list(alpha_fmts))) {
+            ret = AVERROR(ENOMEM);
+            goto fail;
+        }
     if ((ret = ff_formats_ref(main_formats , &ctx->inputs[0]->out_formats)) < 0 ||
         (ret = ff_formats_ref(alpha_formats, &ctx->inputs[1]->out_formats)) < 0 ||
         (ret = ff_formats_ref(main_formats , &ctx->outputs[0]->in_formats)) < 0)
-        return ret;
+            goto fail;
     return 0;
+fail:
+    if (main_formats)
+        av_freep(&main_formats->formats);
+    av_freep(&main_formats);
+    if (alpha_formats)
+        av_freep(&alpha_formats->formats);
+    av_freep(&alpha_formats);
+    return ret;
 }
 
 static int config_input_main(AVFilterLink *inlink)