IPV6 tethering: Check info for null before removeUpstreamV6Interface

When tethering on dual stack networks such as verizon wireless and
sending or recieving MMS you get an null pointer exception and hard
reboot.

It starts to go wrong here:
10-14 19:11:51.375: E/Mms/smil(28035): Failed to parse SMIL document.
10-14 19:11:51.375: E/Mms/smil(28035): org.xml.sax.SAXParseException:
expected start tag notEND_DOCUMENT null@1:1 in java.io.InputStreamReader
@4192ada0

And then:
10-14 19:11:55.559: E/AndroidRuntime(23125): java.lang.NullPointerException
10-14 19:11:55.559: E/AndroidRuntime(23125):    at com.android.server.
connectivity.Tethering$TetherMasterSM$TetherModeAliveState.processMessage
(Tethering.java:1573)

Which is line 1573 of:

https://github.com/CyanogenMod/android_frameworks_base/blob/cm-10.2/services/java/com/android/server/connectivity/Tethering.java#L1573

CMD_UPSTREAM_CHANGED's message.obj can be null (line 776).

Change-Id: I13d531d398f2e33a828c6e81f25f4b2a27518d34