Fix an assertion fault that can only occur if SQLITE_ENABLE_STAT2 is defined

Fix an assertion fault that can only occur if SQLITE_ENABLE_STAT2 is defined
and the constant folding optimization is disabled using sqlite3_test_control().
Problem introduced by [ad8bc68197f2b4] but we missed it prior to the 3.7.4
release due to taking shortcuts and skipping tests in the release checklist.

FossilOrigin-Name: 70a3d81742fcd481f83ccc8f7ff6af58bc29c4fa