Use Arc::get_mut instead of Arc::strong_count == 1 (#555)

Arc::strong_count doesn't come with memory ordering guarantees. As such
it should be possible to destory the context multiple times with the
previous version of this check. 

See rust-lang/rust#117485

Signed-off-by: Greg Morenz <greg-morenz@droid.cafe>

mozjs/src/rust.rs

@@ -428,9 +428,8 @@ impl Runtime {
 impl Drop for Runtime {
     fn drop(&mut self) {
         self.thread_safe_handle.write().unwrap().take();
-        assert_eq!(
-            Arc::strong_count(&self.outstanding_children),
-            1,
+        assert!(
+            Arc::get_mut(&mut self.outstanding_children).is_some(),
             "This runtime still has live children."
         );
         unsafe {