fix(custom/graph): guard signal config with isInt() to avoid SIGABRT

refresh() called config_["signal"].asInt() unconditionally on every RT
signal; a non-integer "signal" value throws Json::LogicError and aborts
Waybar. Matches the guard already present in custom/image/idle_inhibitor.
Fixes #3514.

src/modules/custom_graph.cpp

@@ -139,7 +139,7 @@ void waybar::modules::CustomGraph::waitingWorker() {
 }
 
 void waybar::modules::CustomGraph::refresh(int sig) {
-  if (sig == SIGRTMIN + config_["signal"].asInt()) {
+  if (config_["signal"].isInt() && sig == SIGRTMIN + config_["signal"].asInt()) {
     thread_.wake_up();
   }
 }