client_state: initialize _remote_addr for internal queries

-O1 complains that client_state::_remote_addr is not initialized
(and it is right). The call site is tracing, which likely won't be
invoked for internal queries, but still.
Message-Id: <20180401150410.13651-1-avi@scylladb.com>

service/client_state.hh

@@ -179,6 +179,7 @@ public:
             , _cpu_of_origin(engine().cpu_id())
             , _is_internal(true)
             , _is_thrift(false)
+            , _remote_address(ipv4_addr())
     {}
 
     ///