WEBP: make sure that XMP blob is null-terminated

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@25570 f0d54148-0727-0410-94bb-9a71ac55c965
Fix memleak in error code path

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@25571 f0d54148-0727-0410-94bb-9a71ac55c965