RUIAN: fix double reuse of existing nodes

Existing (near) nodes must be reused only once. Second call
leads to duplicate nodes if there's another near node available.
This can easily happen around a junction point of two or more
inaccurate buildings, e.g. series of garages.

This bug was introduced in cf2d6964534bdf1f4db9679dd52738bfecf61d97.