win32.c: missing initialization

* win32/win32.c (rb_w32_spawn, rb_w32_aspawn_flags): fix missing
  initialization.  pointed out by phasis68 (Heesob Park) at
  [ruby-core:51579].  [Bug #7721]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38909 b2dd03c8-39d4-4d8f-98ff-823fe69b080e