win32.c: extract setup_overlapped and finish_overlapped

* win32/win32.c (setup_overlapped, finish_overlapped): extract from
  rb_w32_read() and rb_w32_write().


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40887 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
win32.c: check error of SetFilePointer

* win32/win32.c (setup_overlapped): check the error code in addition
  to the result of SetFilePointer() to determine if an error occurred,
  because INVALID_SET_FILE_POINTER is a valid value.
  [ruby-core:55098] [Bug #8431]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40888 b2dd03c8-39d4-4d8f-98ff-823fe69b080e