Cast to void* before casting to Node** to make the compiler do no strict-aliasing...

Cast to void* before casting to Node** to make the compiler do no strict-aliasing assumptions, and suppress a warning
if -fstrict-aliasing (enabled with -O2, -O3 and -Os unless forced off as is planned for the time being) is used.
See http://article.gmane.org/gmane.comp.lib.wxwidgets.devel/70262 for details.
Also return NULL instead of 0 in the same GetNodePtr, as the return type is a pointer, not integer.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@37575 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775