remote: fix leaks when matching refspecs

remote: fix leaks when matching refspecs

In `match_explicit()`, we try to match a source ref with a destination
ref according to a refspec item. This matching sometimes requires us to
allocate a new source spec so that it looks like we expect. And while we
in some end up assigning this allocated ref as `peer_ref`, which hands
over ownership of it to the caller, in other cases we don't. We neither
free it though, causing a memory leak.

Fix the leak by creating a common exit path where we can easily free the
source ref in case it is allocated and hasn't been handed over to the
caller.

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