The crash reporter shows a lot of crashes in sqlite3_clear_bindings
which seems to indicate that _stmt is null. We should guard against
a null value in order to avoid crashing.
This should only happen if the prepare call fails. We don't usually
check the return value of the prepare call, but if _stmt is null, the
exec call should return false, not true. We check the result of the
exec call, so this should then abort the sync with an error, rather
than crashing.
In SQLite bindings are not cleared by sqlite3_reset() calls, so
skipping a sqlite3_bind call to create a NULL value doesn't work,
instead the previous value will be written.
To fix this, I clear all bindings in SqlQuery::reset and make sure
to explicitly bind NULL when desired in SqlQuery::bind.
To make sure there's no confusion about SqlQuery::reset and
sqlite3_reset, I rename our method to reset_and_clear_bindings().
