Lost grants when user reference value changed

Here's a problem i'm noticing. I'm not completely convinced this is actually a bug - I'd think someone else would have noticed this by now (maybe i'm totally missing something). But it seems that way.

I have a cck type with a user reference field. When i have a saved grant on a node and then change a user reference value, the grant on the node gets removed. I traced this back to function nodeaccess_insert_userreference, and noticed the logic doesn't check if a user reference field is enabled in the node type settings at /admin/user/nodeaccess. The attached patch adds a check in functions nodeaccess_insert_userreference and nodeaccess_delete_userreference.