gl/wayland: Memory leak when display couldn't connect

When trying to create a wayland display, it may fail because there
is not actually display to connect. It this case NULL is returned
but the created instance is not freed.

This patch unrefs the failed display.

https://bugzilla.gnome.org/show_bug.cgi?id=793483

gst-libs/gst/gl/wayland/gstgldisplay_wayland.c

@@ -130,6 +130,7 @@ gst_gl_display_wayland_new (const gchar * name)
     } else {
       GST_INFO ("Failed to open Wayland display connection.");
     }
+    gst_object_unref (ret);
     return NULL;
   }