Fix missing c_str in trace mode

Author: MaartenS11

src/WARDuino/CallbackHandler.cpp

@@ -194,7 +194,7 @@ Callback::Callback(Module *m, std::string id, uint32_t tidx) {
 
 void Callback::resolve_event(const Event &e) {
     dbg_trace("Callback(%s, %i): resolving Event(%s, \"%s\")\n", topic.c_str(),
-              table_index, e.topic.c_str(), e.payload);
+              table_index, e.topic.c_str(), e.payload.c_str());
 
     // Copy topic and payload to linear memory
     uint32_t start = 10000;  // TODO use reserved area in linear memory