Fix race condition in RTCNetworkMonitor/objc_network_monitor.

Bug: webrtc:13146
Change-Id: I74d645df8847f1e003d1dc8b64e79c6ef9bce285
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/231138
Reviewed-by: Linus Nilsson <lnilsson@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Commit-Queue: Peter Hanspers <peterhanspers@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#34917}

sdk/objc/native/src/objc_network_monitor.mm

@@ -27,6 +27,7 @@ ObjCNetworkMonitor::ObjCNetworkMonitor() {
 }
 
 ObjCNetworkMonitor::~ObjCNetworkMonitor() {
+  [network_monitor_ stop];
   network_monitor_ = nil;
 }
 
@@ -50,6 +51,7 @@ void ObjCNetworkMonitor::Stop() {
     return;
   }
   safety_flag_->SetNotAlive();
+  [network_monitor_ stop];
   network_monitor_ = nil;
   started_ = false;
 }