What

Add `invariant(indexCatalogEntry, str::stream() << "Index '" << indexName << "' not found during dbcheck extra keys scan")` immediately after the `findIndexByName` call at line 459 in `dbcheck.cpp`, before the pointer is dereferenced.

Files: `dbcheck.cpp` (line 459).

Resolves: SERVER-108614

Why

Coverity flags a potential null dereference on the catalog entry pointer. The `invariant` converts the silent null-dereference path into an explicit crash with a diagnostic message, which Coverity recognizes as a handled case. It also documents the assumption that the index must exist at this point in the scan.