graph_blockmodel_covariates.cc: Fix bad_any_cast bug

12 jobs for master
Name Stage Failure
failed
job_gcc_386 Test There is an unknown failure, please try again
  warnings.warn("tight_layout : falling back to Agg renderer")
/usr/lib/python3/dist-packages/matplotlib/tight_layout.py:222: UserWarning: tight_layout : falling back to Agg renderer
warnings.warn("tight_layout : falling back to Agg renderer")
/usr/lib/python3/dist-packages/matplotlib/tight_layout.py:222: UserWarning: tight_layout : falling back to Agg renderer
warnings.warn("tight_layout : falling back to Agg renderer")
/usr/lib/python3/dist-packages/matplotlib/tight_layout.py:222: UserWarning: tight_layout : falling back to Agg renderer
warnings.warn("tight_layout : falling back to Agg renderer")

ERROR: Build failed with: exit status 1
failed
job_gcc_py2_386 Test There is an unknown failure, please try again
Doctest summary
===============
860 tests
1 failure in tests
0 failures in setup code
0 failures in cleanup code
build finished with problems, 137 warnings.

ERROR: Build failed with: exit status 1