Fix SEGFAULT if dns resolv error

Per registry.doRequest, res and client might be nil in case of error
For example, dns resolution errors, /etc/docker/certs.d perms, failed
loading of x509 cert ...
This will make res.StatusCode and res.Body SEGFAULT.

Signed-off-by: Arthur Gautier <baloo@gandi.net>
Merge pull request #7862 from Gandi/bugfixes/segfault-registry-session

Fix SEGFAULT if dns resolv error