Don't crash if mTermSessions == null while handling result from WindowListActivity

Not 100% sure how this happens in the first place, but this should at
least prevent the crash in #142.

Author: steven676

src/jackpal/androidterm/Term.java

@@ -708,7 +708,7 @@ protected void onActivityResult(int request, int result, Intent data) {
                 }
             } else {
                 // Close the activity if user closed all sessions
-                if (mTermSessions.size() == 0) {
+                if (mTermSessions == null || mTermSessions.size() == 0) {
                     mStopServiceOnFinish = true;
                     finish();
                 }