The helper assumes that the n and k values have to be provided as a
single byte push operation, which is only possible for values up to 16.
Fix that by passing the numbers directly to the CScript list, where it's
automatically converted to minimally-encoded pushes (see class
method `CScript.__coerce_instance`, branch `isinstance(other, int)`).
In case of 17..20, this means that the data-pushes are done with two
bytes using OP_PUSH1 (0x01), e.g. for n=20: 0x01,0x14
0c41fc3fa5