compressionparams: force signed compare to avoid compiler warning
compressionparams: force signed compare to avoid compiler warning

GCC detects the <0 comparison with an unsigned and emits a warning.
So cast to a signed integer to silence it.

With this change, our C code is -Wall -Wextra -Wpedantic clean
except for -Wmissing-field-initializers and -Wunused-parameters.
We could fix -Wmissing-field-initializers, but it is a lot of extra
typing to fill in the Python data structures. Most of the
-Wunused-parameters warnings are with regards to extra
PyObject* arguments. We can't just drop those. So I'm fine
with ignoring that warning.
indygreg/python-zstandard,terrelln/python-zstandard,terrelln/python-zstandard,terrelln/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard,terrelln/python-zstandard,indygreg/python-zstandard