test-tool ref-store: fix a memory leak

Fix a memory leak introduced in fa099d2322 (worktree.c: kill
parse_ref() in favor of refs_resolve_ref_unsafe(), 2017-04-24), as a
result we can mark another test as passing with SANITIZE=leak using
"TEST_PASSES_SANITIZE_LEAK=true".

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

t/helper/test-ref-store.c

@@ -96,6 +96,7 @@ static const char **get_store(const char **argv, struct ref_store **refs)
 			die("no such worktree: %s", gitdir);
 
 		*refs = get_worktree_ref_store(*p);
+		free_worktrees(worktrees);
 	} else
 		die("unknown backend %s", argv[0]);