Fixed potential UB

this pointer needs to be duplicated because accepted_versions_ change
e.g. from load_config() could invalidate it

src/server/server.cpp

@@ -657,7 +657,7 @@ void server::read_version(socket_ptr socket, std::shared_ptr<simple_wml::documen
 				   << ":\trejecting them\n";
 
 		// For compatibility with older clients
-		response.set_attr("version", accepted_versions_.begin()->c_str());
+		response.set_attr_dup("version", accepted_versions_.begin()->c_str());
 
 		simple_wml::node& reject = response.root().add_child("reject");
 		reject.set_attr_dup("accepted_versions", utils::join(accepted_versions_).c_str());