auto merge of #13567 : iancormac84/rust/libc_windows_guid_fix, r=alexcrichton

auto merge of #13567 : iancormac84/rust/libc_windows_guid_fix, r=alexcrichton

structure's Data2 and Data3 members expect WORD types instead of DWORD. I
discovered this discrepancy while experimenting with some bindings to
Microsoft's OLE2 api. The discrepancy was corrupting the contents of the
string returned by UuidToString after I used known GUIDs to test the
accuracy of the function binding. I didn't add test cases because it would
mean adding a dependency to my rather incomplete binding library. However,
the fix produces expected string values when tested.