[BUG] stdin poll always return readable

[qzheng527]

Describe the bug

Poll stdin (FD 0) always return readable (Events::IN) even if no input at all.

To reproduce

Steps to reproduce the behavior:

`
/* Watch stdin (fd 0) to see when it has input. */
FD_ZERO(&rfds);
FD_SET(0, &rfds);

/* Wait up to five seconds. */
tv.tv_sec = 5;
tv.tv_usec = 0;

retval = select(1, &rfds, NULL, NULL, &tv);
`
Above code always return immediately.

Expected behavior

No IN event if there is no input.

Environment

• HW: [e.g. SGX1, SGX2]
• OS: [e.g. Ubuntu18.04, CentOS8.1]
• Occlum version: [e.g. 0.17.0]