reftable/basics: handle allocation failures in `reftable_calloc()`
Handle allocation failures in `reftable_calloc()`. While at it, remove our use of `st_mult()` that would cause us to die on an overflow. From the caller's point of view there is not much of a difference between arguments that are too large to be multiplied and a request that is too big to handle by the allocator: in both cases the allocation cannot be fulfilled. And in neither of these cases do we want the reftable library to die. While we could use `unsigned_mult_overflows()` to handle the overflow gracefully, we instead open-code it to further our goal of converting the reftable codebase to become a standalone library that can be reused by external projects. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>