Fix: invalid array allocation in iss_3d.hpp

If OPENMP is not used, the variable threads_ is zero by default. Thus
the array allocation

   Eigen::Vector3d *omp_mem = new Eigen::Vector3d[threads_];

leads to a valid but zero sized array and finally to an invalid write
at line 381-383 (375-377).
Merge pull request #1022 from jgarstka/iss_3d_bugfix

Fix: invalid array allocation in iss_3d.hpp