[wpinet] uv: Stop creating handles when closing loop (#5102)

This prevents EventLoopRunner::Stop() from hanging in the case when
new handles are created after the async walk closes all the handles.

wpinet/src/main/native/cpp/uv/Signal.cpp

@@ -9,6 +9,9 @@
 namespace wpi::uv {
 
 std::shared_ptr<Signal> Signal::Create(Loop& loop) {
+  if (loop.IsClosing()) {
+    return nullptr;
+  }
   auto h = std::make_shared<Signal>(private_init{});
   int err = uv_signal_init(loop.GetRaw(), h->GetRaw());
   if (err < 0) {
@@ -20,6 +23,9 @@ std::shared_ptr<Signal> Signal::Create(Loop& loop) {
 }
 
 void Signal::Start(int signum) {
+  if (IsLoopClosing()) {
+    return;
+  }
   Invoke(
       &uv_signal_start, GetRaw(),
       [](uv_signal_t* handle, int signum) {