When scheduling an AuxilaryTask that hasn't been initialised, try to handle that gracefully

[giuliomoro]

Typical case: someone declared it a global and forgets to create a task on it. Bela_scheduleAuxiliaryTask() is passed a nullptr and segfaults. Can be handled better, e.g.: print an error message and return. I would not necessarily go as far as looking up the pointer in the list of current tasks to see if it's valid, as that could have some undesirable overhead (would it though?).

E.g.: https://forum.bela.io/d/5155-segmentation-fault-error-139-custom-render-mpr121/2