Controller creates NaN when moving into single foot balancing

[mauricefallon]

The controller segfaults when transitioning into single foot balancing. The video should explain it but:

• move mass onto one foot, works fine
• remove other foot from control authority, robot falls. InstQP reports a NaN
  This used to work before the recent upgrade of drake in January.

@tkoolen or @rdeits : could you have a look?
I guess the intention is to replace this with a c++ alternative.

[rdeits]

Ah, ok, that's not good. The C++ switch actually preserves most of the controller code, so this is still likely a relevant issue. I'll look into it now.

[mauricefallon]

And here's the video I mystically referred to:
https://www.dropbox.com/s/mqghzw90krd1cb7/2016-02-fall_single_support-b.m4v?dl=0

[rdeits]

Okay, thanks. I strongly suspect it's related to RobotLocomotion/drake#1603 which I'm in the process of bisecting now