Playground crashes after a compile error

[Vasiliy Tereshkov]

error: /playground_main.um (17, 24): Invalid character literal
status: Exception thrown, see JavaScript console

Uncaught RuntimeError: memory access out of bounds

It can be fixed by replacing

if (thg->umth_frame_callback != -1)

with

if (thg->umka && umkaAlive(thg->umka) && thg->umth_frame_callback != -1)

in frame().