Prevent NullPointerException cases while using SipService.

Some SipService methods may return null, in such cases like no Wi-Fi
connection. Added minimum check to prevent NullPointerExceptions.

Change-Id: Ia7fae57ee893f2564cbfdedb6dc614938ab60ff7
Signed-off-by: Masahiko Endo <masahiko.endo@gmail.com>