Add NULL checks after sqlite3_column_blob in rescore and DiskANN

sqlite3_column_blob() returns NULL for zero-length blobs or on OOM.
Several call sites in rescore KNN and DiskANN node/vector read passed
the result directly to memcpy without checking, risking NULL deref on
corrupt or empty databases. IVF already had proper NULL checks.

Adds corruption regression tests that truncate shadow table blobs and
verify the query errors cleanly instead of crashing.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>