Improve assert checking line index validity in wxGenericListCtrl

Don't just check that the index is not -1, but also that it's valid,
which can catch other problems.

Also replace the ASSERT with CHECK to avoid dereferencing invalid
pointer, it's better to crash when trying to dereference a NULL one.

include/wx/generic/private/listctrl.h

@@ -845,7 +845,7 @@ protected:
     // get the line data for the given index
     wxListLineData *GetLine(size_t n) const
     {
-        wxASSERT_MSG( n != (size_t)-1, wxT("invalid line index") );
+        wxCHECK_MSG( n < m_lines.size(), nullptr, wxT("invalid line index") );
 
         wxListMainWindow *self = wxConstCast(this, wxListMainWindow);