Protect multi-thread access to TSystem::fHelper.
Protect multi-thread access to TSystem::fHelper.

Fix ROOT-9966.

root-mirror/root,root-mirror/root,karies/root,karies/root,olifre/root,karies/root,root-mirror/root,root-mirror/root,root-mirror/root,root-mirror/root,karies/root,root-mirror/root,olifre/root,olifre/root,root-mirror/root,karies/root,karies/root,olifre/root,olifre/root,olifre/root,karies/root,karies/root,olifre/root,olifre/root,olifre/root,karies/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,root-mirror/root,karies/root