web: Correctly check client channel for validity.

It might also be null.

Reviewed-by: Stef Walter <stefw@redhat.com>
web: Use correct client when getting max memory for docker

Previously an error would occur if the run dialog wasn't shown before
the container details page.

Fixes #661
Closes #662
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>