don't access proof after dropping gil (#470)

In `validate_proof` the `proof` variable is accessed after dropping the
GIL. This can result in crashes as python may have released that memory
at any time after the GIL is dropped.

Make sure to use a temp var to hold the proof size so we can avoid
referencing this var

python-bindings/chiapos.cpp

@@ -165,11 +165,12 @@ PYBIND11_MODULE(chiapos, m)
 
                 std::string proof_str(proof);
                 const uint8_t *proof_ptr = reinterpret_cast<const uint8_t *>(proof_str.data());
+                auto proof_size = proof_str.size();
 
                 LargeBits quality;
                 {
                     py::gil_scoped_release release;
-                    quality = v.ValidateProof(seed_ptr, k, challenge_ptr, proof_ptr, len(proof));
+                    quality = v.ValidateProof(seed_ptr, k, challenge_ptr, proof_ptr, proof_size);
                 }
                 if (quality.GetSize() == 0) {
                     return stdx::optional<py::bytes>();