This bug meant that setting a parameter in a parent class for a doubly
nested parameter group, where the parameter index in the parent class
is 4 or greater would actually set the first element in that parent
class.
At the moment only one parameter fits these narrow constraints - the
RCn_DZ element of the RC_Channel_aux class. So if someone set RC5_DZ
to 17 in ArduPlane it would actually set RC5_MIN to 17.