Decorate GDALGCPsToGeoTransform() and GDALInvGeoTransform() with CPL_WARN_UNUSED_RESULT annotation, and fix related warning in the code base

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@26281 f0d54148-0727-0410-94bb-9a71ac55c965