This was a regression in 0bb81a43c9 which was not keeping an edge case in mind if a weak pointer is locked during object destruction. This issue was fixed by 525a05ea2b1d00a1db901f883f1755943aeff33c, but that is not enough since it causes CharacterJointEffect::get_character() to return null now, so we need a separate method to check if a CharacterJointEffect belongs to a given character. This is more efficient, too.
Fixes#330