Audience settings removed when saving the node.

Groupstructure:
-a
--b
Both groups are private groups.

User A is member of of group --b
User AB is member of group -a and --b
User B is member of group --b

1. AB creates a node for audience groups -a and --ab.
2. B can access the node.
3. A edits the body text (not the audience settings) and saves it.
4. B can no longer access the node.

When saving a node, the access settings of non-member groups are removed. They should be ignored and remain unchanged.