api_lib: Prevent netconn_write when sendmbox is closed

[42dot-bongjun-hur]

adds the following code to netconn_write_vectors_partly:

if (conn->flags & NETCONN_FLAG_MBOXCLOSED) {
  /* don't write if closed: this might block the application task
     waiting on sendmbox forever! */
  return ERR_CONN;
}

I think, it prevents writing when the send mailbox is closed, avoiding a potential deadlock.