Bypass asserts for `XMLDocument::Value()`

When the node is an XMLDocument, bypass calling the `_value.GetStr()` function, since we know we have to return `( const char* )0` inevitably.

This fixes #323
diff --git a/tinyxml2.cpp b/tinyxml2.cpp
index 6e1985b..e6bb328 100755
--- a/tinyxml2.cpp
+++ b/tinyxml2.cpp
@@ -645,6 +645,8 @@
 

 const char* XMLNode::Value() const 

 {

+    if ( this->ToDocument() )

+        return ( const char* )0;

     return _value.GetStr();

 }