Subtle bug in data_object class

(This is more of an issue in Totara, which makes extensive use of data_object as a base class, but it could have an effect in Moodle as well, so I'm reporting here).

Inside data_object::fetch_all_helper, the $params array is reused, instead of generating a new $params array.

This means that any values that do not match a known field (required_fields or optional_fields) will cause a mismatch in the query.

So, assuming we have a subclass of data_object (I'll call it example_object), with the following fields:
field1, field2, field3

If I write:
$x = new example_object(['invalidfield' => 1, 'field1' => 5, 'field2' => 4]);

Then, fetch_all_helper will translate the $params array into:

$params = ['invalidfield' => 1, 'field1' => 5, 'field2' => 4, 0 => 5, 1 => 4];
$wheresql = 'field1 = ? AND field2 = ?';

Internally, get_records_select() will notice there are too many params, and truncate them to:
$params = ['invalidfield' => 1, 'field1' => 5];
and then to:
$params = [0 => 1, 1 => 5];

Resulting in a final query that looks like:
"... WHERE field1 = 1 AND field2 = 5"

Which is not correct.

This has the potential to cause all sorts of subtle bugs in the Moodle code (especially when 3rd-parties are reusing the data_object class - which is something I've seen on a number of occasions - despite the reservations expressed in MDL-22186)