Fix potential crash in KCmFontInst on invalid index
Summary:
If !index.isValid() grp remains null and any use of it will crash.
There is a check for that, but then there's code relying on grp being
valid both before and after that check.
Test Plan: Compiles
Reviewers: Plasma, mart
Reviewed By: mart
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D4394