Using bridges or switching to bridges sometimes does not work with tor 0.3.2

Trying to switch from a direct connection to using a bridge is not working anymore with tor on master (found while using commit f2f1cab2) on my Linux box.

Steps to reproduce:

1. Take a recent Tor Browser (e.g. an alpha version)
2. Make sure you replace tor shipped in your Tor Browser instance with one compiled from a recent master commit
3. Start Tor Browser and choose direct connection
4. Shut Tor Browser down after you got greeted with the about:tor page
5. Restart Tor Browser and press the "Open Settings" button before the bootstrapping is finished
6. Select e.g. the recommended obfs4 default bridge option
7. The start-up stalls (I quit Tor Browser after 5 minutes waiting)

The first bad commit is c21cfd28 which fixed legacy/trac#17750 (moved). Without that one Tor Browser resumes bootstrapping after a couple of seconds and is using an obfs4 bridge.