kdump: Fix crash on undefined config
kdump: Fix crash on undefined config

`this.props.kdumpStatus.{target,config}` gets set from the config
parser, which is independent from the systemd unit settings in
`this.props.kdumpStatus`. This could lead to the latter already being
available, while the former was still pending. The page would then show
a clickable "Loading" link which caused a "TypeError: Cannot convert
undefined or null to object " crash.

Robustify this by only setting targetCanChange to true when we
explicitly know that it's possible to do so.

mvollmer/cockpit,cockpit-project/cockpit,mvollmer/cockpit,cockpit-project/cockpit,martinpitt/cockpit,martinpitt/cockpit,martinpitt/cockpit,martinpitt/cockpit,mvollmer/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,mvollmer/cockpit,mvollmer/cockpit,cockpit-project/cockpit,martinpitt/cockpit