Prevent RED.stop being called multiple times if >1 signal received

packages/node_modules/node-red/red.js

@@ -448,10 +448,14 @@ httpsPromise.then(function(startupHttps) {
         process.exit(1);
     });
 
+    var stopping = false;
     function exitWhenStopped() {
-        RED.stop().then(function() {
-            process.exit();
-        });
+        if (!stopping) {
+            stopping = true;
+            RED.stop().then(function() {
+                process.exit();
+            });
+        }
     }
     process.on('SIGINT', exitWhenStopped);
     process.on('SIGTERM', exitWhenStopped);