remote: check config validity before creating rewrite struct

remote: check config validity before creating rewrite struct

When parsing url.foo.insteadOf, we call make_rewrite() and only then
check to make sure the config value is a string (and return an error if
it isn't). This isn't quite a leak, because the struct we allocate is
part of a global array, but it does leave a funny half-finished struct.

In practice, it doesn't make much difference because we exit soon after
due to the config error anyway. But let's flip the order here to avoid
leaving a trap for somebody in the future.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>