win pipe: missing conditional check on uv_pipe_zero_readfile_thread_proc

fixes a race condition where if uv_read_stop was called shortly after uv_read_start or a successful read and before the uv_pipe_zero_readfile_thread_proc thread started, that thread would call the blocking ReadFile call after the HANDLE_READING flag had already been cleared

src/win/pipe.c

@@ -1089,6 +1089,7 @@ static DWORD WINAPI uv_pipe_zero_readfile_thread_proc(void* parameter) {
     uv_mutex_unlock(m);
   }
 restart_readfile:
+  if (handle->flags & UV_HANDLE_READING) {
       result = ReadFile(handle->handle,
                         &uv_zero_,
                         0,
@@ -1110,6 +1111,9 @@ static DWORD WINAPI uv_pipe_zero_readfile_thread_proc(void* parameter) {
              }
           }
       }
+  } else {
+      result = 1; /* successfully aborted read before it even started */
+  }
   if (hThread) {
     assert(hThread == handle->readfile_thread);
     /* mutex does not control clearing readfile_thread */