Don't ping subspace_storage mod if master connection is down

The system for preventing items being sent without a connection to the
master and (and thus voiding the items) is to only do so if a ping from
the plugin has been received in the last 5 seconds of in-game time.
Don't send pings without being fully connected to the master to make
sure this system actually activates during connection downtime.

plugins/subspace_storage/instance.js

@@ -27,6 +27,9 @@ class InstancePlugin extends libPlugin.BaseInstancePlugin {
 
 	async onStart() {
 		this.pingId = setInterval(() => {
+			if (!this.slave.connected) {
+				return; // Only ping if we are actually connected to the master.
+			}
 			this.sendRcon(
 				"/sc __subspace_storage__ global.ticksSinceMasterPinged = 0", true
 			).catch(err => this.unexpectedError(err));