r8627: fixed a big memory leak in the spnego gensec code in session

r8627: fixed a big memory leak in the spnego gensec code in session
setup. Andrew, please check over this.

What happens is this:

 - run the BASE-SECLEAK test
 - with each failed session setup using spnego a gensec ctx is leaked into the smb_conn structure
 - after the client disconnects these are finally cleaned up as they
   are all children of the connection structure
 - the cleanup of the millions of memory objects takes long enough
   that the next operation in test_posix.sh sometimes fails with a timeout

Andrew, can you also look at the talloc_reference() on line 332 of
sesssetup.c ? I suspect it isn't needed (I don't think it does any
actual harm though)
(This used to be commit b40fb6a4569ccc1fa1750a1e534e18a020764b4c)