thread.c: suppress a warning
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62305 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
thread.c: suppress a warning
false positive maybe-uninitialized warning by gcc 7 and 8.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62305 b2dd03c8-39d4-4d8f-98ff-823fe69b080e