Bug970187, CDMA-3way calls

https://bugzilla.mozilla.org/show_bug.cgi?id=970187
If it does not checking the activeCall() is null, add-call-button is　visible on the end of call screen in conference call.
![conference_end_in_cdma_not_ac_check](https://cloud.githubusercontent.com/assets/7699864/3226192/8896ca10-f067-11e3-8210-6c4b6a2a6357.png)