BUG/MINOR: config: fix a crash on startup when a disabled backend references a peer

Disabled backends don't have their symbols resolved. We must not initialize
their peers section since they're not valid and instead still contain the
section's name.

There are other places where such unions are still in use, and other similar
errors might still happen. Ideally we should get rid of all of them in the
quite sensible config stage.
1 file changed