selinux: fix memory leak in policydb_init()

Since roles_init() adds some entries to the role hash table, we need to
destroy also its keys/values on error, otherwise we get a memory leak in
the error path.

Cc: <stable@vger.kernel.org>
Reported-by:  <syzbot+fee3a14d4cdf92646287@syzkaller.appspotmail.com>
Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>