win32.c: no error after retry

* win32/win32.c (waitpid): return 0 on error after retried.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41450 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
win32.c: GetLastError once

* win32/win32.c (poll_child_status): call GetLastError() once.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41451 b2dd03c8-39d4-4d8f-98ff-823fe69b080e