Fix segfaults with ArcSDE BLOB/CLOB/NCLOB fields

[aalbarello]

The supplied patch fixes random segfaults and weird SDE error messages that I've been getting on several layers with NCLOB fields. It appears that the appropriate SE_BLOB_INFO/SE_CLOB_INFO/SE_NCLOB_INFO struct needs to be completely zeroed out before calling SE_stream_get_*lob (not that there's anything wrong with zeroing them out, it's good practice anyway, it's just weird that SDE would be doing anything with them anyway before filling them in).