Adds additional check, avoids undefined error.

Needs an additional check for available devices after merging https://github.com/jitsi/lib-jitsi-meet/pull/139.

JitsiMediaDevices.js

      */
     setAudioOutputDevice: function (deviceId) {
 
-        if (RTC.getCurrentlyAvailableMediaDevices().length > 0)
+        if (RTC.getCurrentlyAvailableMediaDevices()
+            && RTC.getCurrentlyAvailableMediaDevices().length > 0)
         {
             // if we have devices info report device to stats
             // normally this will not happen on startup as this method is called