Auto merge of #19654 - emilio:geckolib-nightly, r=jdm

style: Allow building stylo with rust nightly.

See individual commits for details.

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/19654)
<!-- Reviewable:end -->

components/style/gecko/wrapper.rs

@@ -1787,7 +1787,7 @@ impl<'le> Eq for GeckoElement<'le> {}
 impl<'le> Hash for GeckoElement<'le> {
     #[inline]
     fn hash<H: Hasher>(&self, state: &mut H) {
-        (self.0 as *const _).hash(state);
+        (self.0 as *const RawGeckoElement).hash(state);
     }
 }
 

components/style/gecko_bindings/sugar/ownership.rs

@@ -109,7 +109,6 @@ pub unsafe trait HasArcFFI : HasFFI {
     ///
     /// &GeckoType -> &Arc<ServoType>
     fn as_arc<'a>(ptr: &'a &Self::FFIType) -> &'a RawOffsetArc<Self> {
-        debug_assert!(!(ptr as *const _).is_null());
         unsafe {
             transmute::<&&Self::FFIType, &RawOffsetArc<Self>>(ptr)
         }