`OneOfDescriptor_each` is registered as a Ruby method which takes zero
parameters, which means it should take one argument.

When Ruby invokes `OneOfDescriptor_each`, it calls it with one parameter
only, which is one less than what `OneOfDescriptor_each` takes before
this commit. Calling a function with the wrong number of argument is
technically undefined behavior.

See also: §6.5.2.2, N1256