Micro optimization in wxDisplay ctor

Don't check for the index validity if it's 0, which is the most common
case, as index 0 is always valid and so we can avoid calling GetCount(),
which might have a non-trivial cost, completely.

src/common/dpycmn.cpp

@@ -115,7 +115,7 @@ wxIMPLEMENT_DYNAMIC_CLASS(wxDisplayModule, wxModule);
 
 wxDisplay::wxDisplay(unsigned n)
 {
-    wxASSERT_MSG( n < GetCount(),
+    wxASSERT_MSG( n == 0 || n < GetCount(),
                     wxT("An invalid index was passed to wxDisplay") );
 
     m_impl = Factory().CreateDisplay(n);