HFP: Check if name is nullptr before calling GetStringUTFChars

* The name parameter in phoneStateChangeNative() is optional and can be
  null
* We need to check whether it is null before calling GetStringUTFChars

Test: make, connect to BT car kits
Change-Id: Iad4be23f8ca1408742e04c37844f82c46b86c086
1 file changed