correct handling of roads with more than 250 points

For roads with multiple segments, the lastSegment 
bit was set too late, so that all segments
were written with the lastSegment bit set to true 
(the default value).

git-svn-id: http://svn.mkgmap.org.uk/mkgmap/trunk@2977 25d90789-57f7-4ee0-8453-03a3dfeeeb22