cppcheck:
Avoid 'operator=' should return reference to 'this' instance.

Change-Id: Iac146fe31b2cebc8e4f4c74d79ad98625db04637
Reviewed-on: https://gerrit.libreoffice.org/69004
Tested-by: Jenkins
Reviewed-by: Julien Nabet <serval2412@yahoo.fr>