* process.c (rb_f_system): set last_status when status == -1 because
	  there is no path to set it on win32. this patch is derived from
	  [ruby-core:16787], submitted by Luis Lavena <luislavena at gmail.com>


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_8_6@17307 b2dd03c8-39d4-4d8f-98ff-823fe69b080e