submodule--helper: fix "errmsg_str" memory leak

Fix a memory leak introduced in e83e3333b57 (submodule: port submodule
subcommand 'summary' from shell to C, 2020-08-13), we sometimes append
to the "errmsg", and need to free the "struct strbuf".

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>
