Dialer: Fix NPE in CallStats

Change-Id: I0eed1e66f803e072c58d85c78c253730f81f029d
See: https://jira.cyanogenmod.org/browse/BUGDUMP-114773?jql=text%20~%20%22Callstats%22
(cherry picked from commit 526bd53bc3d1ba938abf4bd5f9a6906f9c1d479e)