win32.c: optimization

* win32/win32.c (poll_child_status): trivial optimization.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40565 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
win32.c: fix build error

* win32/win32.c (poll_child_status): fix build error on older mingw.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40566 b2dd03c8-39d4-4d8f-98ff-823fe69b080e