check-ref-format: fix trivial memory leak

check-ref-format: fix trivial memory leak

Fix a memory leak in "git check-ref-format" that's been present in the
code in one form or another since 38eedc634bc (git check-ref-format
--print, 2009-10-12), the code got substantially refactored in
cfbe22f03f9 (check-ref-format: handle subcommands in separate
functions, 2010-08-05).

As a result we can mark a test as passing with SANITIZE=leak using
"TEST_PASSES_SANITIZE_LEAK=true".

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>