gstreamer/element: Use from_glib_none() for the release_pad() trampoline

The reference to the pad we get passed is not guaranteed to be valid for
the whole scope of release_pad(). It might be the last reference as
owned by GstElement, and gst_element_remove_pad() would destroy it then.

gstreamer/src/subclass/element.rs

@@ -373,7 +373,7 @@ unsafe extern "C" fn element_release_pad<T: ObjectSubclass>(
     let wrap: Element = from_glib_borrow(ptr);
 
     gst_panic_to_error!(&wrap, &instance.panicked(), (), {
-        imp.release_pad(&wrap, &from_glib_borrow(pad))
+        imp.release_pad(&wrap, &from_glib_none(pad))
     })
 }