[Tile-serving] [osm2pgsql] Incorrect multi backend linestring geoms (#274)

[Paul Norman]

So, this is a strange one. Using the above style which creates a linestring for any way, I started with the  relation/full for the [BC 91](https://www.openstreetmap.org/relation/417823) and started deleting ways to get to a smaller test case. Being a result from cgimap I made sure to use `--slim`.

Eventually with an XML file with all the nodes and two ways (35354811, 35354763) I had 35354763 import with the wrong geometry. I then deleted all other nodes, renumbered them, and got the following XML, which reproduces it.

```xml
<osm version="0.6">
 <node id="1" lat="49.0919077" lon="-122.8938177"/>
 <node id="2" lat="49.0925635" lon="-122.8941386"/>
 <node id="3" lat="49.1694335" lon="-122.9990449"/>
 <node id="4" lat="49.1694438" lon="-122.9983216"/>
 <node id="5" lat="49.0922320" lon="-122.8939654"/>
 <way id="1">
  <nd ref="1"/>
  <nd ref="5"/>
  <nd ref="2"/>
 </way>
 <way id="2">
  <nd ref="3"/>
  <nd ref="4"/>
 </way>
</osm>
```

Some other observations
* Switching the node list between the ways changes if the bug occurs, but switching only IDs does not
* This is independent of `--slim` but `--slim` makes it easier to look at the middle storage.


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https://github.com/openstreetmap/osm2pgsql/issues/274#issuecomment-72740009
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