fix possible error message in RestrictionRelation: ... internal error: via way doesn't contain expected points

This message was printed when a via way with additional nodes was
added multiple times to the road network, the restriction was ignored.
This is now handled correctly.
Sample: http://www.openstreetmap.org/way/8104544

git-svn-id: http://svn.mkgmap.org.uk/mkgmap/trunk@3295 25d90789-57f7-4ee0-8453-03a3dfeeeb22