return 1 after timer_add(...) breaks modules

[cyriax0]

Reproducing:

In an gfx_* module (e.g. gfx_rainbow)

int draw(int argc, char* argv[]) {
    [...]
    frame++;
    nexttick += FRAMETIME;
    timer_add(nexttick, modno, 0, NULL);
    if (frame > 100) return 1;
    return 0;
}

What happens

After returning 1 modules the timers are broken and modules start a switch loop in which the last two modules are switched rapidly without waiting for timers or fininshing. The output is erratic. No modules run normally.

Possible solutions

clear timer for that module after a module returns 1