Suppress various harmless warnings in MinGW build with -Wconversion.

No real changes, simply add casts to make the implicit conversions that g++
warns about when using -Wconversion explicit.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67964 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/textctrl.cpp

@@ -1125,7 +1125,7 @@ void wxTextCtrl::AppendText(const wxString& text)
     // don't do this if we're frozen, saves some time
     if ( !IsFrozen() && IsMultiLine() && GetRichVersion() > 1 )
     {
-        ::SendMessage(GetHwnd(), WM_VSCROLL, SB_BOTTOM, NULL);
+        ::SendMessage(GetHwnd(), WM_VSCROLL, SB_BOTTOM, (LPARAM)NULL);
     }
 #endif // wxUSE_RICHEDIT
 }