House numbers, edge cases

Including zero. However a zero does not show up on the map. But it otherwise
works - number 2 is a little further down the road as you would expect - it just
doesn't display the number zero. In all or most parts of the world zero is not
a valid house number, so this is more about preventing a crash on invalid data.

git-svn-id: http://svn.mkgmap.org.uk/mkgmap/trunk@2499 25d90789-57f7-4ee0-8453-03a3dfeeeb22