[C++] Fix crash for missing token after #include directive.

Change-Id: Ia54c5ddc801962c89dc0483f5912f1de436b8f43
Reviewed-by: Erik Verbruggen <erik.verbruggen@nokia.com>

src/libs/cplusplus/pp-engine.cpp

@@ -1130,6 +1130,11 @@ void Preprocessor::handleIncludeDirective(PPToken *tk)
     }
     included = included.trimmed();
 
+    if (included.isEmpty()) {
+        //### TODO: error message
+        return;
+    }
+
 //    qDebug("include [[%s]]", included.toUtf8().constData());
     Client::IncludeType mode;
     if (included.at(0) == '"')