windows: initialize stop_flag explicitly

The default loop lives in the bss section so it's zeroed on startup
but loops created with uv_loop_new() live on the heap and contain
random garbage. Initialize the stop_flag explicitly to avoid spurious
bugs.

src/win/core.c

@@ -116,6 +116,7 @@ static void uv_loop_init(uv_loop_t* loop) {
   loop->active_udp_streams = 0;
 
   loop->timer_counter = 0;
+  loop->stop_flag = 0;
 
   loop->last_err = uv_ok_;
 }