uncomment validation of Tokenizer::_tokensBack

lib/token.cpp

@@ -142,7 +142,7 @@ void Token::replace(Token *replaceThis, Token *start, Token *end)
     start->previous(replaceThis->previous());
     end->next(replaceThis->next());
 
-    if (end->tokensBack && *(end->tokensBack) == replaceThis)
+    if (end->tokensBack && *(end->tokensBack) == end)
     {
         while (end->next())
             end = end->next();

lib/tokenize.cpp

@@ -5126,13 +5126,11 @@ bool Tokenizer::validate() const
         return false;
     }
 
+    // Validate that the Tokenizer::_tokensBack is updated correctly during simplifications
     if (lastTok != _tokensBack)
     {
-        // TODO, the two lines below should be uncommented and
-        // problems that appear with testrunner should be fixed
-
-        //cppcheckError(lastTok);
-        //return false;
+        cppcheckError(lastTok);
+        return false;
     }
 
     return true;