Fix crash when using HTML help in 64 bit wxMSW build

HtmlHelp() function takes 64 bit ULONG_PTR, not 32 bit DWORD, in 64 bits, see
https://msdn.microsoft.com/en-us/library/windows/desktop/bb762267.aspx and
passing it a wrongly sized pointer resulted in a crash.

Closes https://github.com/wxWidgets/wxWidgets/pull/239

docs/changes.txt

@@ -48,6 +48,13 @@ Changes in behaviour which may result in build errors
 - WXWIN_OS_DESCRIPTION doesn't exist any longer, use wxGetOsDescription().
 
 
+3.1.1: (not released yet)
+----------------------------
+
+wxMSW:
+
+- Fix crash when using wxCHMHelpController() in 64 bit builds (Xlord2).
+
 3.1.0: (released 2016-02-29)
 ----------------------------