Fixing crash (project-chip#36343)

Author: woody-apple

src/darwin/Framework/CHIP/MTRDeviceConnectivityMonitor.mm

@@ -120,6 +120,10 @@ - (void)handleResolvedHostname:(const char *)hostName port:(uint16_t)port error:
 {
     std::lock_guard lock(sConnectivityMonitorLock);
 
+    if (hostName == NULL) {
+        MTR_LOG_ERROR("%@ NULL host resolved, ignoring", self);
+        return;
+    }
     // dns_sd.h: must check and call deallocate if error is kDNSServiceErr_ServiceNotRunning
     if (error == kDNSServiceErr_ServiceNotRunning) {
         MTR_LOG_ERROR("%@ disconnected from dns-sd subsystem", self);