clone: fix memory leak in wanted_peer_refs()

clone: fix memory leak in wanted_peer_refs()

Fix a memory leak added in 0ec4b1650cc (clone: fix ref selection in
--single-branch --branch=xxx, 2012-06-22).

Whether we get our "remote_head" from copy_ref() directly, or with a
call to guess_remote_head() it'll be the result of a copy_ref() in
either case, as guess_remote_head() is a wrapper for copy_ref() (or it
returns NULL).

We can't mark any tests passing passing with SANITIZE=leak using
"TEST_PASSES_SANITIZE_LEAK=true" as a result of this change, but
e.g. "t/t1500-rev-parse.sh" now gets closer to passing.

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