Fix crash in wxDC::GetMultiLineTextExtent() after last commit.

Don't call wxTextMeasure::DoGetTextExtent() with NULL width pointer, it now
supposes that both width and height pointers are non-NULL.

Add at least a trivial unit test for GetMultiLineTextExtent().

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72700 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/textmeasurecmn.cpp

@@ -101,7 +101,8 @@ void wxTextMeasureBase::GetMultiLineTextExtent(const wxString& text,
                 if ( !heightLineDefault )
                 {
                     // but we don't know it yet - choose something reasonable
-                    DoGetTextExtent(wxS("W"), NULL, &heightLineDefault);
+                    int dummy;
+                    DoGetTextExtent(wxS("W"), &dummy, &heightLineDefault);
                 }
 
                 heightTextTotal += heightLineDefault;