Crash on startup if previous PortAudio output device not available

Running Mac OS X Mavericks on a MacBook Pro, I can crash the latest nightly build of Musescore by changing the output device in PortAudio and then disconnecting that device. Musescore will always crash on startup.

Here's the sequence of events to reproduce it:
* Connect HDMI TV to Mac laptop as 2nd display
* In Preferences, I/O tab, set PortAudio output device to HDMI
* Restart Musescore for change to take effect
* Have a jam session. Everything working correctly.
* Quit Musescore
* Disconnect HDMI TV
* Reopen Musescore. It repeatedly crashes at startup. You won't be able to get it to run.
* Reconnect HDMI TV. You can now run Musescore, select "built-in output device" in PortAudio and normal operation is restored.