sqlite: fix use-after-free in Exec() and ApplyChangeset()

When sqlite3_exec() or sqlite3changeset_apply() call JavaScript
callbacks (user-defined functions, conflict handlers, or filter
callbacks), the DatabaseSync object could be garbage-collected
if the JavaScript code drops all references to it. Both methods
only held a raw DatabaseSync* pointer on the C++ stack, which
V8 GC does not track.

Add a BaseObjectPtr<DatabaseSync> guard that keeps the database
alive for the duration of these SQLite API calls, preventing a
use-after-free when the JavaScript callback triggers GC.

Signed-off-by: Matteo Collina <hello@matteocollina.com>
PR-URL: #64535
Reviewed-By: Edy Silva <edigleyssonsilva@gmail.com>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: René <contact.9a5d6388@renegade334.me.uk>

Author: mcollina