[krbdev.mit.edu #5876] init_rtree() assertion triggered in kvno

[Greg Hudson via RT]

After #6966, the TGS code path uses k5_client_realm_path, which 
absolutely cannot return less than two elements, so I'm pretty sure this 
bug can't happen any more.

#7668 points out a case where krb5_walk_realm_tree can return a one-
element list, so that may help explain how this crash could have happened 
with the old code.