Try to ensure that the configuration is valid before returning it

This is an attempt to avoid running into (IMO incorrect) assert's in
KConfigGroup's read* functions when having kdelibs built in debug mode.
Unfortunately KConfig* has no other API to tell it that it should create
a KConfigGroup thats been fetched, other than causing a filesystem access
to sync.

So this trades crashing KDevelop with one doing potentially more fs syncs.

I couldn't verify wether this fixes the issue since I don't have a
debug-kdelibs anywhere.