fix(formatter): keep links with formatted text in MFM posts

The email autolink clause also matched a link whose only child is an
element, raised on "mailto:" <> element, and the rescue turned the
whole post into escaped source text.

lib/pleroma/formatter.ex

@@ -169,7 +169,8 @@ defmodule Pleroma.Formatter do
     Enum.map(nodes, &strip_email_autolinks(&1, emails))
   end
 
-  defp strip_email_autolinks({"a", attrs, [email], _meta} = node, emails) do
+  defp strip_email_autolinks({"a", attrs, [email], _meta} = node, emails)
+       when is_binary(email) do
     if List.keyfind(attrs, "href", 0) == {"href", "mailto:" <> email} and
          MapSet.member?(emails, email) do
       email

test/pleroma/formatter_test.exs

@@ -375,6 +375,16 @@ defmodule Pleroma.FormatterTest do
   end
 
   describe "markdown_to_html/2" do
+    test "keeps formatted link text when stripping email autolinks" do
+      opts = %{breaks: true, strip_email_autolinks: true}
+
+      assert Formatter.markdown_to_html("[**hello**](https://example.org/)", opts) ==
+               ~s(<p><a href="https://example.org/"><strong>hello</strong></a></p>)
+
+      assert Formatter.markdown_to_html("[`code`](https://example.org/)", opts) ==
+               ~s(<p><a href="https://example.org/"><code class="inline">code</code></a></p>)
+    end
+
     test "escapes link attribute values" do
       markdown = ~s|[click](http://example.com/?a=x" onerror="alert(1))|