[gtkmm] Gtk::MediaStream: Fix get_error()

[Kjell Ahlstedt <kjellahl src gnome org>]

commit f26da35d00f8d5c6acc4013b94a6635c3f0246d1
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Fri Jan 18 10:50:32 2019 +0100

    Gtk::MediaStream: Fix get_error()
    
    Don't use g_error_copy(). It prints a critical message if
    gtk_media_stream_get_error() returns a nullptr.

 gtk/src/mediastream.hg | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/gtk/src/mediastream.hg b/gtk/src/mediastream.hg
index d42236ff..89f66823 100644
--- a/gtk/src/mediastream.hg
+++ b/gtk/src/mediastream.hg
@@ -55,7 +55,7 @@ class MediaStream : public Glib::Object, public Gdk::Paintable
 
 public:
   _WRAP_METHOD(bool is_prepared() const, gtk_media_stream_is_prepared)
-#m4 _CONVERSION(`const GError*',`Glib::Error',`Glib::Error(g_error_copy($3))')
+#m4 _CONVERSION(`const GError*',`Glib::Error',`Glib::Error(const_cast<GError*>($3), true)')
   _WRAP_METHOD(Glib::Error get_error() const, gtk_media_stream_get_error)
   _WRAP_METHOD(bool has_audio() const, gtk_media_stream_has_audio)
   _WRAP_METHOD(bool has_video() const, gtk_media_stream_has_video)