tree_insert() allows duplicate entries, leading to usertable corruption and authentication failures

[kartiksubbarao]

I recently noticed intermittent authentication failures that I tracked down to duplicate user entries in the usertable. For example, this mavis_lookup_final() logic often tries to add duplicates, which tree_insert() is intended to stop. The problem is that the duplicate checking doesn't happen until the new node is already positioned in the RB tree, by which time some duplicates could have already been missed. (e.g. insert A, B, A causes the duplicate A to be added).

Fortunately, the fix is simple -- just move the duplicate detection a few lines up to the positioning section. I'll submit a pull request.

[kartiksubbarao]

pull request: #12