Comment embeds: fix reflow flicker in firefox

Firefox was returning a height of 0 when an iframe was hidden.
This was causing unnecessary reflows when toggling between the
options for comment embeds. There doesn't seem to be a good
reason to broadcast a 0 height up to the parent window, so
i've added a guard for that.

r2/r2/public/static/js/embed/embed.js

@@ -7,7 +7,7 @@
   function checkHeight() {
     var height = document.body.clientHeight;
 
-    if (App.height !== height) {
+    if (height && App.height !== height) {
       App.height = height;
 
       App.postMessage(window.parent, 'resize', height, '*');