os: TestReadWriteDeadlineRace failure on linux-amd64-clang builder

[bcmills]

There was a single failure of TestReadWriteDeadlineRace on the linux-amd64-clang builder at head:

--- FAIL: TestReadWriteDeadlineRace (0.00s)
    timeout_test.go:498: Read returned non-timeout error read |0: not pollable
FAIL
FAIL	os	0.638s

I've only seen this failure once, but recording it here in case it becomes a pattern.

2019-11-20T22:38:34-39a9cb4/linux-amd64-clang

CC @ianlancetaylor

[ianlancetaylor]

This means that epoll returned EPOLLERR as an event for the file descriptor in question. I don't really see how that is possible here.