Conference calling crash on null call object

After conference call ends there are some NPE happening because the call
is being accessed after getting destroyed. Check that underlying call
object is not NULL

Bug: b/34983172
Test: Hangup conference call from CarKitt
Change-Id: I08bfc5478a4dce4e608d7dc5cf9688b65fbc6637
diff --git a/android/app/src/com/android/bluetooth/hfpclient/connserv/HfpClientConnection.java b/android/app/src/com/android/bluetooth/hfpclient/connserv/HfpClientConnection.java
index 2a30ec8..749bb75 100644
--- a/android/app/src/com/android/bluetooth/hfpclient/connserv/HfpClientConnection.java
+++ b/android/app/src/com/android/bluetooth/hfpclient/connserv/HfpClientConnection.java
@@ -114,7 +114,7 @@
     }
 
     public boolean inConference() {
-        return mAdded && mCurrentCall.isMultiParty() &&
+        return mAdded && mCurrentCall != null && mCurrentCall.isMultiParty() &&
                 getState() != Connection.STATE_DISCONNECTED;
     }