Running with USE_MIXED_ADDITION causes segfault

[imeckler]

Comping with mixed-addition (as enabled by the USE_MIXED_ADDITION CMake flag) causes the Groth16 prover to segfault.

One can reproduce by running the following:

mkdir build && cd build
cmake .. -DMULTICORE=ON -DUSE_MIXED_ADDITION=ON -DCURVE=BN128
make -j12 profile_r1cs_gg_ppzksnark
./libsnark/profile_r1cs_gg_ppzksnark 1024 1 Fr