win: fix msvc warning

PR-URL: https://github.com/libuv/libuv/pull/2091
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jameson Nash <vtjnash@gmail.com>

src/win/tty.c

@@ -360,6 +360,8 @@ int uv_tty_set_mode(uv_tty_t* tty, uv_tty_mode_t mode) {
     }
   } else {
     was_reading = 0;
+    alloc_cb = NULL;
+    read_cb = NULL;
   }
 
   uv_sem_wait(&uv_tty_output_lock);