Fix bad-free in ensure_vector_match: aCleanup(a) → aCleanup(*a)

When the second vector argument fails to parse, the cleanup of the
first vector was called with the double-pointer 'a' instead of '*a'.
When the first vector was parsed from JSON text (cleanup = sqlite3_free),
this called sqlite3_free on a stack address, causing a crash.

Found by the vec-mismatch fuzz target.

Shout out to @renatgalimov in #257 for finding the original bug!

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>