Handle wxELLIPSIZE_NONE gracefully in wxControl::Ellipsize()

Since 44bcc3a723 wxRendererGeneric::DrawItemText()
could call Ellipsize() with wxELLIPSIZE_NONE mode, resulting in an assert.

Fix this by just not doing anything in this case.

Closes #125.

interface/wx/control.h

@@ -383,7 +383,8 @@ public:     // static functions
             wxDC::GetPartialTextExtents() function.
         @param mode
             The ellipsization mode. This is the setting which determines
-            which part of the string should be replaced by the ellipsis.
+            which part of the string should be replaced by the ellipsis
+            (unless it is ::wxELLIPSIZE_NONE in which case nothing is done).
             See ::wxEllipsizeMode enumeration values for more info.
         @param maxWidth
             The maximum width of the returned string in pixels.