remote: fix leaking peer ref when expanding refmap

remote: fix leaking peer ref when expanding refmap

When expanding remote refs via the refspec in `get_expanded_map()`, we
first copy the remote ref and then override its peer ref with the
expanded name. This may cause a memory leak though in case the peer ref
is already set, as this field is being copied by `copy_ref()`, as well.

Fix the leak by freeing the peer ref before we re-assign the field.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>