submodule--helper: check repo{_submodule,}_init() return values

submodule--helper: check repo{_submodule,}_init() return values

Fix code added in ce125d431aa (submodule: extract path to submodule
gitdir func, 2021-09-15) and a77c3fcb5ec (submodule--helper: get
remote names from any repository, 2022-03-04) which failed to check
the return values of repo_init() and repo_submodule_init(). If we
failed to initialize the repository or submodule we could segfault
when trying to access the invalid repository structs.

Let's also check that these were the only such logic errors in the
codebase by making use of the "warn_unused_result" attribute. This is
valid as of GCC 3.4.0 (and clang will catch it via its faking of
__GNUC__ ).

As the comment being added to git-compat-util.h we're piggy-backing on
the LAST_ARG_MUST_BE_NULL version check out of lazyness. See
9fe3edc47f1 (Add the LAST_ARG_MUST_BE_NULL macro, 2013-07-18) for its
addition. The marginal benefit of covering gcc 3.4.0..4.0.0 is
near-zero (or zero) at this point. It mostly matters that we catch
this somewhere.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>