reftable/stack: fix parameter validation when compacting range

The `stack_compact_range()` function receives a "first" and "last" index
that indicates which tables of the reftable stack should be compacted.
Naturally, "first" must be smaller than "last" in order to identify a
proper range of tables to compress, which we indeed also assert in the
function. But the validations happens after we have already allocated
arrays with a size of `last - first + 1`, leading to an underflow and
thus an invalid allocation size.

Fix this by reordering the array allocations to happen after we have
validated parameters. While at it, convert the array allocations to use
the newly introduced macros.

Note that the relevant variables pointing into arrays should also be
converted to use `size_t` instead of `int`. This is left for a later
commit in this series.

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

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