Fixes crash (#452)

* fix(JingleSessionPC): do not log whole object
* fix(JitsiConference): fix a crash

The code will crash if there's neither P2P nor JVB session yet.

JitsiConference.js

     } else {
         ssrc = track.getSSRC();
     }
-    if (!container.id || !ssrc) {
+    if (!container.id || !ssrc || !peerConnection) {
         return;
     }
 

modules/xmpp/JingleSessionPC.js

         // Do something with reason and reasonCondition when we start to care
         // this.reasonCondition = reasonCondition;
         // this.reasonText = reasonText;
-        logger.info('Session terminated', this, reasonCondition, reasonText);
+        logger.info(`Session terminated ${this}`, reasonCondition, reasonText);
 
         this.close();
     }
                     && this.peerconnection.connectionState !== 'closed'))
             && this.peerconnection.close();
     }
+
+    /**
+     * Converts to string with minor summary.
+     * @return {string}
+     */
+    toString() {
+        return `JingleSessionPC[p2p=${this.isP2P},`
+                    + `initiator=${this.isInitiator},sid=${this.sid}]`;
+    }
 }