checkout,clone: check return value of create_symref

checkout,clone: check return value of create_symref

It's unlikely that we would fail to create or update a
symbolic ref (especially HEAD), but if we do, we should
notice and complain. Note that there's no need to give more
details in our error message; create_symref will already
have done so.

While we're here, let's also fix a minor memory leak in
clone.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>