compressor: change populate_cdict() to no-op
compressor: change populate_cdict() to no-op

Rather than checking preconditions in the caller, let's just have
the function do this.
terrelln/python-zstandard,terrelln/python-zstandard,terrelln/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard,terrelln/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard