Fix a bug when frameState is null which occurs in offline mobile devices

While testing OL3 with an ApplicationCache manifest in offline mode, I found that `frameState` can be `null` in  `Map::handlePostRender`.  There is already an `if` for this but one line that relies on `frameState` being valid is outside this code branch.  With this change, there are no further javascript errors on iOS in offline/airplane mode.