Protect against stream being null in connection.close.

This may happen if we close a connection before getting any tracks.

static/sfu.js

@@ -56,12 +56,14 @@ Connection.prototype.close = function(sendit) {
     while(this.timers.length > 0)
         clearInterval(this.timers.pop());
 
+    if(this.stream) {
         this.stream.getTracks().forEach(t => {
             try {
                 t.stop();
             } catch(e) {
             }
         });
+    }
     this.pc.close();
 
     if(sendit) {