Break retain cycle and fix bug

src/objective-c/GRPCClient/private/GRPCConnectivityMonitor.m

@@ -153,14 +153,18 @@ static void PassFlagsToContextInfoBlock(SCNetworkReachabilityRef target,
 
 - (void)handleLossWithHandler:(void (^)())handler
       wifiStatusChangeHandler:(nonnull void (^)())wifiStatusChangeHandler {
+  __weak typeof(self) weakSelf = self;
   [self startListeningWithHandler:^(GRPCReachabilityFlags *flags) {
+    typeof(self) strongSelf = weakSelf;
+    if (strongSelf) {
       if (!flags.reachable) {
         handler();
-    } else if (!_previousReachabilityFlags ||
+      } else if (strongSelf->_previousReachabilityFlags &&
-               (flags.isWWAN ^ _previousReachabilityFlags.isWWAN)) {
+                 (flags.isWWAN ^ strongSelf->_previousReachabilityFlags.isWWAN)) {
         wifiStatusChangeHandler();
       }
-    _previousReachabilityFlags = flags;
+      strongSelf->_previousReachabilityFlags = flags;
+    }
   }];
 }