reftable/basics: fix segfault when growing `names` array fails

reftable/basics: fix segfault when growing `names` array fails

When growing the `names` array fails we would end up with a `NULL`
pointer. This causes two problems:

  - We would run into a segfault because we try to free names that we
    have assigned to the array already.

  - We lose track of the old array and cannot free its contents.

Fix this issue by using a temporary variable. Like this we do not
clobber the old array that we tried to reallocate, which will remain
valid when a call to realloc(3P) fails.

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