config.c: don't leak memory in handle_path_include()

Fix a memory leak in the error() path in handle_path_include(), this
allows us to run t1305-config-include.sh under SANITIZE=leak,
previously 4 tests there would fail. This fixes up a leak in
9b25a0b52e0 (config: add include directive, 2012-02-06).

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