Never return 0 for stream protocol

On socket disconnection, on Linux, recv() returns -1 and errno is set.
But on Windows, errno is 0.

In that case, AVERROR(errno) == 0, leading to the warning:

> Invalid return value 0 for stream protocol

To avoid the problem, if errno is 0, return AVERROR_EOF.

Ref: commit 2876463d394c3bc4dc239a605e65ebab75598353