games/kajaani-kombat: unbreak with clang 4.0

client_net.cpp:34:12: error: ordered comparison between pointer and zero ('SDL_cond *' and 'int')

if (cond <0)
    ~~~~ ^~

client_net.cpp:40:13: error: ordered comparison between pointer and zero ('SDL_mutex *' and 'int')

if (mutex < 0)
    ~~~~~ ^ ~

client_net.cpp:48:16: error: ordered comparison between pointer and zero ('SDL_Thread *' and 'int')

if (trans_th < 0)
    ~~~~~~~~ ^ ~

client_net.cpp:54:16: error: ordered comparison between pointer and zero ('SDL_Thread *' and 'int')

if (input_th < 0)
    ~~~~~~~~ ^ ~

game_server.cpp:67:14: error: ordered comparison between pointer and zero ('SDL_Thread *' and 'int')

if (con_th < 0)
    ~~~~~~ ^ ~

Reported by: antoine (via exp-run)