promisor-remote: fix possible issue when no URL is advertised

promisor-remote: fix possible issue when no URL is advertised

In the 'KnownUrl' case, in should_accept_remote(), let's check that
`remote_url` is not NULL before we use strcmp() to compare it with
the local URL. This could avoid crashes if a server starts to not
advertise any URL in the future.

If `remote_url` is NULL, we should reject the URL. Let's also warn in
this case because we warn otherwise when a remote is rejected to try
to help diagnose things at the end of the function.

And while we are checking that remote_url is not NULL and warning if
it is, it makes sense to also help diagnose the case where remote_url
is empty.

Also while at it, let's spell "URL" with uppercase letters in all the
warnings.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>