alloc-util: whenever any of our alloca() wrappers is used to allocate overly...

alloc-util: whenever any of our alloca() wrappers is used to allocate overly large memory blocks, hit an assert()

Of course, this should never happen, but let's better be safe than
sorry, and abort rather than continue when a too large memory block is
allocated, simply asa safety precaution.

An early abort is better than continuing with a likely memory corruption
later.