Crash when adding `junction=roundabout` to a node

[taylor.smock]

First present in r15889.

Steps to reproduce:
1) Open a new layer
2) Add a node
3) Select the node
4) Open the add tag dialog
5) Enter junction into the key field
6) Start typing roundabout into the value field (it will crash on r)

Unless someone is already working on it, I'll try to get a patch attached as soon as possible.