Ensure we have crypto before accessing it in Room model

If crypto startup has failed, we shouldn't try to access any of its methods.
This fixes a variant of this in the `Room` model.

spec/unit/room.spec.js

@@ -1318,6 +1318,9 @@ describe("Room", function() {
                     // events should already be MatrixEvents
                     return function(event) {return event;};
                 },
+                isCryptoEnabled() {
+                    return true;
+                },
                 isRoomEncrypted: function() {
                     return false;
                 },

src/models/room.js

@@ -496,7 +496,7 @@ Room.prototype.loadMembersIfNeeded = function() {
     const inMemoryUpdate = this._loadMembers().then((result) => {
         this.currentState.setOutOfBandMembers(result.memberEvents);
         // now the members are loaded, start to track the e2e devices if needed
-        if (this._client.isRoomEncrypted(this.roomId)) {
+        if (this._client.isCryptoEnabled() && this._client.isRoomEncrypted(this.roomId)) {
             this._client._crypto.trackRoomDevices(this.roomId);
         }
         return result.fromServer;