reftable: fix allocation count on realloc error

reftable: fix allocation count on realloc error

When realloc(3) fails, it returns NULL and keeps the original allocation
intact.  REFTABLE_ALLOC_GROW overwrites both the original pointer and
the allocation count variable in that case, simultaneously leaking the
original allocation and misrepresenting the number of storable items.

parse_names() avoids the leak by keeping the original pointer if
reallocation fails, but still increase the allocation count in such a
case as if it succeeded.  That's OK, because the error handling code
just frees everything and doesn't look at names_cap anymore.

reftable_buf_add() does the same, but here it is a problem as it leaves
the reftable_buf in a broken state, with ->alloc being roughly twice as
big as the actually allocated memory, allowing out-of-bounds writes in
subsequent calls.

Reimplement REFTABLE_ALLOC_GROW to avoid leaks, keep allocation counts
in sync and still signal failures to callers while avoiding code
duplication in callers.  Make it an expression that evaluates to 0 if no
reallocation is needed or it succeeded and 1 on failure while keeping
the original pointer and allocation counter values.

Adjust REFTABLE_ALLOC_GROW_OR_NULL to the new calling convention for
REFTABLE_ALLOC_GROW, but keep its support for non-size_t alloc variables
for now.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>