Add a test case for issue #323

When compiled in "debug mode", this test case verifies that an assert is fired, when `XMLDocument::Value()` is called.
diff --git a/xmltest.cpp b/xmltest.cpp
index b70fc51..4325cb7 100644
--- a/xmltest.cpp
+++ b/xmltest.cpp
@@ -1460,7 +1460,21 @@
 		XMLTest( "Error should be cleared", false, doc.Error() );

 	}

 

-	// ----------- Performance tracking --------------

+        {

+                // No matter - before or after successfully parsing a text -

+                // calling XMLDocument::Value() causes an assert in debug.

+                const char* validXml = "<?xml version=\"1.0\" encoding=\"utf-8\" ?>"

+                                       "<first />"

+                                       "<second />";

+                XMLDocument* doc = new XMLDocument();

+                const char* value;

+                XMLTest( "XMLDocument::Value() fires assert?", NULL, doc->Value() );

+                doc->Parse( validXml );

+                XMLTest( "XMLDocument::Value() fires assert?", NULL, doc->Value() );

+                delete doc;

+        }

+

+        // ----------- Performance tracking --------------

 	{

 #if defined( _MSC_VER )

 		__int64 start, end, freq;