Added nullptr check for pthradpool_get_threads_count (#34087)
Summary:
We get seg fault without this in using XNNPACK.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34087
Differential Revision: D20199787
Pulled By: kimishpatel
fbshipit-source-id: d3d274e7bb197461632b21688820cd4c10dcd819
