reftable/stack: guard against NULL list_file in stack_destroy

reftable/stack: guard against NULL list_file in stack_destroy

When reftable_new_stack() fails partway through initialization
(e.g., reftable_buf_addstr returns an OOM error before
reftable_buf_detach assigns p->list_file), it jumps to the error
path which calls reftable_stack_destroy(p). At that point,
p->list_file is still NULL because the detach never happened.

reftable_stack_destroy() passes st->list_file unconditionally to
read_lines(), which calls open(filename, O_RDONLY). Passing NULL
to open() is undefined behavior and will typically crash.

Guard the read_lines() call with a NULL check on st->list_file.
When list_file is NULL, there are no table files to clean up
anyway, so skipping read_lines is the correct behavior.

Pointed out by Coverity.

Assisted-by: Claude Opus 4.6
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>