Appease static analyzer warning about null string passed to strlen
https://bugs.webkit.org/show_bug.cgi?id=110658

Reviewed by Benjamin Poulain.

Add a null check before calling strlen

* wtf/Threading.cpp:
(WTF::createThread):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@144020 268f45cc-cd09-0410-ab3c-d52691b4dbfc