avoid dereferencing invalidated iterator 

calling m_start_nodes.erase(sit) invalidates ```sit```, I get 

```bash
./runtest.sh: line 4: 44719 Segmentation fault: 11  ./osmcoastline --debug --verbose --overwrite --output-lines --output-polygons=both --output-rings -o testdata.db testdata.osm
```
on os x.