[Merkaartor] Crash on merging streets

[Norbert Wenzel]

Hi,

I just found a reproducable error when merging streets of different 
highway type. I currently add data which has been previously mapped by 
Yahoo images, so most of the streets aren't classified or connected 
correctly. When merging a street which consists of two different roads 
in the OSM data at the moment, the merging creates a street like
    highway=residential;living_street
which is perfectly fine.

When I try to change the highway type to the correct classification 
Merkaartor crashes reproducable.

Problem encountered with rev. 12302 under Ubuntu.

cheers,
Norbert