Merge pull request #13180 from muxi/fix-objc-msan

Fix threading problem in gRPC ObjC

src/objective-c/GRPCClient/GRPCCall.m

@@ -268,8 +268,10 @@ static NSString *const kBearerPrefix = @"Bearer ";
 // method.
 // TODO(jcanizales): Rename to readResponseIfNotPaused.
 - (void)startNextRead {
-  if (self.state == GRXWriterStatePaused) {
+  @synchronized(self) {
-    return;
+    if (self.state == GRXWriterStatePaused) {
+      return;
+    }
   }
 
   dispatch_async(_callQueue, ^{