Adding lock and reset call_ in ~ClientContext to avoid race condition (#30270)

* Adding lock and reset call_ in ~ClientContext to avoid race condition

* Remove lock in ~ClientContext

src/cpp/client/client_context.cc

@@ -79,6 +79,7 @@ ClientContext::ClientContext()
 ClientContext::~ClientContext() {
   if (call_) {
     grpc_call_unref(call_);
+    call_ = nullptr;
   }
   g_client_callbacks->Destructor(this);
 }