GPKG: enforce read-only mode

git-svn-id: https://svn.osgeo.org/gdal/trunk@27083 f0d54148-0727-0410-94bb-9a71ac55c965
GPKG: fix crash when the SRS cannot be found

git-svn-id: https://svn.osgeo.org/gdal/trunk@27084 f0d54148-0727-0410-94bb-9a71ac55c965