reftable/basics: stop using `st_mult()` in array allocators

reftable/basics: stop using `st_mult()` in array allocators

We're using `st_mult()` as part of our macro helpers that allocate
arrays. This is bad due two two reasons:

  - `st_mult()` causes us to die in case the multiplication overflows.

  - `st_mult()` ties us to the Git codebase.

Refactor the code to instead detect overflows manually and return an
error in such cases.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>