unhandled case in TrapezoidParted

[rayvburn]

After a corrupted rel_loc (-nan), which occurs after Cn's multiplier is set to a negative value, following was caught:

terminate called after throwing an instance of 'std::runtime_error'
  what():  unhandled case in trapezoid update (section 2)

Of course this is related to some abnormal parameter configuration and should be fixed, but for now will leave it as it is.