Fix Cdma conference cleanup bug.

The conference pointer in the CdmaConferenceController wasn't being cleared after the
conference was destroyed.  This prevented subsequent conferences from being created
correctly (ie. couldn't merge calls into a new conference).

Bug: 19355781, 18612712
Change-Id: I0612d7e2f89cdb56ed330289d08ee4e7ef007970
diff --git a/src/com/android/services/telephony/CdmaConferenceController.java b/src/com/android/services/telephony/CdmaConferenceController.java
index 3d90fd3..8a26365 100644
--- a/src/com/android/services/telephony/CdmaConferenceController.java
+++ b/src/com/android/services/telephony/CdmaConferenceController.java
@@ -189,6 +189,7 @@
             if (mConference != null) {
                 Log.i(this, "Destroying the CDMA conference connection.");
                 mConference.destroy();
+                mConference = null;
             }
         }
     }