OGRMILayerAttrIndex::LoadConfigFromXML(): fix potential crash when pszMetadataFilename was null (related to #4031)

git-svn-id: https://svn.osgeo.org/gdal/trunk@28134 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_mingw_w64