style: Avoid writing into the empty array header.

Differential Revision: https://phabricator.services.mozilla.com/D55291

components/style/gecko_bindings/sugar/ns_t_array.rs

@@ -89,9 +89,11 @@ impl<T> nsTArray<T> {
     pub unsafe fn set_len(&mut self, len: u32) {
         // this can leak
         debug_assert!(len >= self.len() as u32);
+        if self.len() == len as usize {
+            return;
+        }
         self.ensure_capacity(len as usize);
-        let header = self.header_mut();
-        header.mLength = len;
+        self.header_mut().mLength = len;
     }
 
     /// Resizes an array containing only POD elements
@@ -103,6 +105,9 @@ impl<T> nsTArray<T> {
     where
         T: Copy,
     {
+        if self.len() == len as usize {
+            return;
+        }
         self.ensure_capacity(len as usize);
         let header = self.header_mut();
         header.mLength = len;