fix(JitsiConference): a crash

Avoid a crash if for whatever reason there is no JVB JingleSessionPC at
the time when P2P is being stopped.

Add FIXME about possible situation where Jicofo invite will arrive after
the P2P has been established already.

JitsiConference.js

@@ -2023,6 +2023,9 @@ JitsiConference.prototype._onIceConnectionEstablished
 
     // Stop media transfer over the JVB connection
     if (this.jvbJingleSession) {
+        // FIXME if for whatever reason invite from Jicofo for the JVB
+        // connection arrives, after the P2P has been established
+        // this needs to be called as well.
         this._suspendMediaTransferForJvbConnection();
     }
 
@@ -2295,7 +2298,9 @@ JitsiConference.prototype._stopP2PSession
 
     // Swap remote tracks, but only if the P2P has been fully established
     if (wasP2PEstablished) {
-        this._resumeMediaTransferForJvbConnection();
+        if (this.jvbJingleSession) {
+            this._resumeMediaTransferForJvbConnection();
+        }
 
         // Remove remote P2P tracks
         this._removeRemoteP2PTracks();