telephony: Handle NPE in setDataEnabledUsingSubId()

This should fix the following crash:

E AndroidRuntime: *** FATAL EXCEPTION IN SYSTEM PROCESS: main
E AndroidRuntime: java.lang.RuntimeException: Error receiving broadcast Intent { act=android.net.wifi.STATE_CHANGE flg=0x44000010 (has extras) } in com.android.server.ProfileTriggerHelper@229e56b3
...
E AndroidRuntime: Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'void com.android.internal.telephony.ITelephony.setDataEnabled(int, boolean)' on a null object reference
E AndroidRuntime: 	at android.telephony.TelephonyManager.setDataEnabledUsingSubId(TelephonyManager.java:3845)

Change-Id: I046ff7e5e2b8de1717a461c08813100b10fcc6e5