af_unix: Return immediately when manage_oob() returns NULL for 0-length buffer.

In the Linux kernel, the following vulnerability has been resolved: af_unix: Return immediately when manage_oob() returns NULL for 0-length buffer. Fahad Alharbi reported that recv(0, MSG_PEEK) triggers busy-wait in unix_stream_read_generic() if recv() is blocking and the last skb in the queue is MSG_OOB skb. In such a situation, TCP returns 0 immediately regardless of blocking or non-blocking. Let's follow the behaviour.

Where it's fixed