workgraph_fib test seg-faults with threads backend and hwloc

[ndellingwood]

The workgraph_fib unit test is seg-faulting when using hwloc/1.11.3 on SKX and KNL architectures with the Threads backend.

KNL:
intel/compilers/18.0.128, hwloc/1.11.3, cmake/3.12.3
intel/compilers/17.0.098, hwloc/1.11.3, cmake/3.12.3
intel/compilers/17.1.132, hwloc/1.11.3, cmake/3.12.3
gcc/6.2.0, hwloc/1.11.3, cmake/3.12.3
gcc/7.1.0, hwloc/1.11.3, cmake/3.12.3

SKX:
intel/compilers/19.1.144, hwloc/1.11.8, cmake/3.12.3

Sample configure reproducer:

export HWLOC_ROOT=<PATH_TO_HWLOC>

../generate_makefile.bash \
    --arch=KNL \
    --cxxflags="-Wall -Wshadow -pedantic -Werror -Wsign-compare -Wtype-limits -Wuninitialized" \
    --with-hwloc=$HWLOC_ROOT \
    --compiler=icpc \
    --with-devices=Pthread,Serial