Convert num_alloc to a size_t in stack.c and tweak style

We were casting num_alloc to size_t in lots of places, or just using it in
a context where size_t makes more sense - so convert it. This simplifies
the code a bit.

Also tweak the style in stack.c a bit following on from the previous
commit

Reviewed-by: Rich Salz <rsalz@openssl.org>