Fix bug in window message handling
Fix bug in window message handling

Fix a bug on windows (8?) where a WM_GETMINMAXINFO event is send befor
the WM_NCCREATE event resulting in a null pointer as a result of the
GetWindowLong( hwnd, GWL_USERDATA ) call. Solution: add a nullpointer
check befor calling a member function of window.

migulorama/OOGL,migulorama/OOGL