x11/antimicro: unbreak with clang 4.0

src/mainwindow.cpp:367:19: error: ordered comparison between pointer and zero ('QMap<SDL_JoystickID, InputDevice *> *' (aka 'QMap<int, InputDevice *> *') and 'int')

if (joysticks > 0)
    ~~~~~~~~~ ^ ~

PR: 216358
Reported by: antoine (via exp-run)