Backport missing null check in "adb get-state".

Bug: https://code.google.com/p/android/issues/detail?id=184011
Bug: http://b/23709037
Change-Id: Ia33814bd2adc310c2471cd6a12b9a73f27d77182
diff --git a/adb/adb.cpp b/adb/adb.cpp
index f64b19f..c09aee3 100644
--- a/adb/adb.cpp
+++ b/adb/adb.cpp
@@ -919,7 +919,7 @@
     if(!strncmp(service,"get-state",strlen("get-state"))) {
         transport = acquire_one_transport(CS_ANY, ttype, serial, NULL);
         SendOkay(reply_fd);
-        SendProtocolString(reply_fd, transport->connection_state_name());
+        SendProtocolString(reply_fd, transport ? transport->connection_state_name() : "unknown");
         return 0;
     }
 #endif // ADB_HOST