Fixed (1) handling of engine gimbal vectoring and (b) a divide by zero bug in SteeringHelper::Inverse().

(1) is required for the Flight Computer to properly work on vessels without reaction wheels (i.e. when playinv with the RealismOverhaul mod). This bug can be easily reproduced by adding the following .cfg file and using the Flight Computer on the Stayputnik probe core:
@PART[probeCoreSphere]
{
  !MODULE[ModuleReactionWheel]
  {}
}

(2) manifested itself during debugging of (1) and should be an obvious fix.