don't crash on malformed avatar and banner values

weird values in href will cause base64 encoding to fail later down the
line, so let's make sure the value we're passing on is somewhat sane, or
at the very least a binary

this fixes #482

lib/pleroma/user.ex

@@ -366,21 +366,21 @@ def invisible?(%User{invisible: true}), do: true
   def invisible?(_), do: false
 
   def avatar_url(user, options \\ []) do
-    case user.avatar do
-      %{"url" => [%{"href" => href} | _]} ->
-        href
-
-      _ ->
-        unless options[:no_default] do
-          Config.get([:assets, :default_user_avatar], "#{Endpoint.url()}/images/avi.png")
-        end
-    end
+    default = Config.get([:assets, :default_user_avatar], "#{Endpoint.url()}/images/avi.png")
+    do_optional_url(user.avatar, default, options)
   end
 
   def banner_url(user, options \\ []) do
-    case user.banner do
-      %{"url" => [%{"href" => href} | _]} -> href
-      _ -> !options[:no_default] && "#{Endpoint.url()}/images/banner.png"
+    do_optional_url(user.banner, "#{Endpoint.url()}/images/banner.png", options)
+  end
+
+  defp do_optional_url(field, default, options \\ []) do
+    case field do
+      %{"url" => [%{"href" => href} | _]} when is_binary(href) ->
+        href
+
+      _ ->
+        unless options[:no_default], do: default
     end
   end
 

test/pleroma/user_test.exs

@@ -2509,6 +2509,16 @@ test "avatar fallback" do
     assert User.avatar_url(user, no_default: true) == nil
   end
 
+  test "avatar object with nil in href" do
+    user = insert(:user, avatar: %{"url" => [%{"href" => nil}]})
+    assert User.avatar_url(user) != nil
+  end
+
+  test "banner object with nil in href" do
+    user = insert(:user, banner: %{"url" => [%{"href" => nil}]})
+    assert User.banner_url(user) != nil
+  end
+
   test "get_host/1" do
     user = insert(:user, ap_id: "https://lain.com/users/lain", nickname: "lain")
     assert User.get_host(user) == "lain.com"