Language protocol: corrected start position of diagnostic if start was placed at end of buffer

src/source_language_protocol.cc

@@ -891,7 +891,7 @@ void Source::LanguageProtocolView::update_diagnostics(std::vector<LanguageProtoc
         if(!end.is_end())
           end.forward_char();
         else
-          start.forward_char();
+          start.backward_char();
       }
 
       bool error = false;