Add GDALDataset::ValidateRasterIOOrAdviseReadParameters() to make validation of common input parameters of RasterIO() and AdivseRead(); use it in ECW driver and make that latter accept panBandMap == NULL without segfaulting

git-svn-id: https://svn.osgeo.org/gdal/trunk@26082 f0d54148-0727-0410-94bb-9a71ac55c965