core: always initialize ExecParamters.bus_endpoint_fd to -1

Otherwise it might be passed in as 0, which is a valid fd, but usually
does not refer to a real endpoint.

src/core/socket.c

@@ -1398,6 +1398,7 @@ static int socket_spawn(Socket *s, ExecCommand *c, pid_t *_pid) {
                 .apply_permissions = true,
                 .apply_chroot      = true,
                 .apply_tty_stdin   = true,
+                .bus_endpoint_fd   = -1,
         };
 
         assert(s);