Make VRTXXXXXSource::RasterIO() robust to psExtraArg = NULL, as called by the PostGISRaster driver

git-svn-id: https://svn.osgeo.org/gdal/trunk@28073 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx