Fix some races due to promises completing after we've switched rooms

Add a few isMounted() checks to promise handlers so that we don't end up
throwing NPEs.

src/components/structures/RoomView.js

@@ -353,11 +353,14 @@ module.exports = React.createClass({
     _paginateCompleted: function() {
         debuglog("paginate complete");
 
-        this.setState({
-            room: MatrixClientPeg.get().getRoom(this.props.roomId)
-        });
+        // we might have switched rooms since the paginate started - just bin
+        // the results if so.
+        if (!this.isMounted()) return;
 
-        this.setState({paginating: false});
+        this.setState({
+            room: MatrixClientPeg.get().getRoom(this.props.roomId),
+            paginating: false,
+        });
     },
 
     onSearchResultsFillRequest: function(backwards) {
@@ -535,7 +538,7 @@ module.exports = React.createClass({
 
         return searchPromise.then(function(results) {
             debuglog("search complete");
-            if (!self.state.searching || self.searchId != localSearchId) {
+            if (!this.isMounted() || !self.state.searching || self.searchId != localSearchId) {
                 console.error("Discarding stale search results");
                 return;
             }

src/components/structures/ScrollPanel.js

@@ -158,6 +158,10 @@ module.exports = React.createClass({
 
     // check the scroll state and send out backfill requests if necessary.
     checkFillState: function() {
+        if (!this.isMounted()) {
+            return;
+        }
+
         var sn = this._getScrollNode();
 
         // if there is less than a screenful of messages above or below the
@@ -346,6 +350,12 @@ module.exports = React.createClass({
      * message panel.
      */
     _getScrollNode: function() {
+        if (!this.isMounted()) {
+            // this shouldn't happen, but when it does, turn the NPE into
+            // something more meaningful.
+            throw new Error("ScrollPanel._getScrollNode called when unmounted");
+        }
+
         var panel = ReactDOM.findDOMNode(this.refs.geminiPanel);
 
         // If the gemini scrollbar is doing its thing, this will be a div within