Sddm-kcm should depend on sddm.
Sddm sets up an sddm user which is used to run the greeter.
Therefore it should not be possible to run sddm-kcm without an sddm user existing.
But if it should happen that's the case, this patch adds a check for the sddm user.
If the user is not present, everything will be aborted and an error message will pop-up.