Fix ValidateRasterIOOrAdviseReadParameters() to avoid a segfault on MapServer wcs_20_multi_getcov_trim_y_to_1px.tif test (trunk only)

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@26511 f0d54148-0727-0410-94bb-9a71ac55c965