Problem: curve_client_t may emit misleading event on bad data processed by curve_client_t::decode

Solution: use check_basic_command_structure in curve_client_t::decode, also prepare other client mechanisms to use that method by rearranging inheritance hierarchy

src/stream_engine.cpp

@@ -698,7 +698,8 @@ bool zmq::stream_engine_t::handshake ()
                 mechanism = new (std::nothrow)
                     gssapi_server_t (session, peer_address, options);
             else
-                mechanism = new (std::nothrow) gssapi_client_t (options);
+                mechanism =
+                    new (std::nothrow) gssapi_client_t (session, options);
             alloc_assert (mechanism);
         }
 #endif