thread.c: stop if forked in a sub-thread
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45899 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
thread.c: stop if forked in a sub-thread
the thread has become the main thread.
[ruby-core:62070] [Bug #9751]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45899 b2dd03c8-39d4-4d8f-98ff-823fe69b080e