Ensure existing attributes are not leaked on attribute insertion
diff --git a/tinyxml2.cpp b/tinyxml2.cpp
index 96f3ad7..6fd9363 100755
--- a/tinyxml2.cpp
+++ b/tinyxml2.cpp
@@ -1717,9 +1717,11 @@
         attrib = CreateAttribute();

         TIXMLASSERT( attrib );

         if ( last ) {

+            TIXMLASSERT( last->_next == 0 );

             last->_next = attrib;

         }

         else {

+            TIXMLASSERT( _rootAttribute == 0 );

             _rootAttribute = attrib;

         }

         attrib->SetName( name );

@@ -1777,9 +1779,11 @@
             // avoids re-scanning the attribute list. Preferring performance for

             // now, may reconsider in the future.

             if ( prevAttribute ) {

+                TIXMLASSERT( prevAttribute->_next == 0 );

                 prevAttribute->_next = attrib;

             }

             else {

+                TIXMLASSERT( _rootAttribute == 0 );

                 _rootAttribute = attrib;

             }

             prevAttribute = attrib;