Prevent error when clicking 'log in'

If you joined a room before clicking 'log in', it would throw an
exception here and break.

src/components/views/rooms/MessageComposer.js

@@ -67,8 +67,10 @@ export default class MessageComposer extends React.Component {
     }
 
     componentWillUnmount() {
+        if (MatrixClientPeg.get()) {
             MatrixClientPeg.get().removeListener("event", this.onEvent);
         }
+    }
 
     onEvent(event) {
         if (event.getType() !== 'm.room.encryption') return;