Fix js error when changing your own username (bug #779694)

Change-Id: I642c1a8f8cf36b2fc3423dfed477dbfda27005c8
Signed-off-by: Richard Mansfield <richard.mansfield@catalyst.net.nz>

htdocs/account/index.php

@@ -314,7 +314,9 @@ function clearPasswords(form, data) {
     }
     if (data.username) {
         var username = getFirstElementByTagAndClassName('a', null, 'profile-sideblock-username');
-        replaceChildNodes(username, data.username);
+        if (username) {
+            replaceChildNodes(username, data.username);
+        }
     }
 }
 ");