Initialize member variable in constructor (might fix the two first Valgrind warning in https://github.com/mapserver/mapserver/issues/4558 although I'm not sure how that could have happened)

git-svn-id: https://svn.osgeo.org/gdal/trunk@26486 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_mingw