Fix crash in ModifyLayoutItemCommand when m_item is NULL

Happens in release-mode. Not clear yet why. In any case, all
of the code does handle the case m_item is null except
updateText(). This commit adjusts updateText() so we at
least not crash.

Change-Id: I7511e901e925cf23c18632ab2c21ae15f9ce8d3b
Reviewed-on: https://codereview.kdab.com/14161
Reviewed-by: Continuous Integration <build@kdab.com>
Reviewed-by: Volker Krause <volker@kdab.com>