Add null checks for getImsCall()

When certain RTT operations are performed near the end of a call, the
phone process may crash with a NPE since mImsCall in ImsPhoneConnection
becomes null. Add null checks to fix this.

Change-Id: I4da899f26ce897224429387927a48288454d0c8b
Merged-In: I4da899f26ce897224429387927a48288454d0c8b
Fixes: 110570770
Test: manual