db: enforce that bindings be done in order.

This is almost always true already; fix up the few non-standard ones.

This is enforced with an assert, and I ran the entire test suite to
double-check.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

db/utils.c

@@ -81,6 +81,7 @@ static struct db_stmt *db_prepare_core(struct db *db,
 	stmt->query = db_query;
 	stmt->executed = false;
 	stmt->inner_stmt = NULL;
+	stmt->bind_pos = -1;
 
 	tal_add_destructor(stmt, db_stmt_free);