s3: Attempt to fix bug 7518

s3: Attempt to fix bug 7518

If select returns -1, we can't rely on the fd sets. The current code might loop
endlessly because when putting an invalid fd (the closed socket?) on the read
set, a select implementation might choose not to touch it but directly return
with EINVAL. Thus run_events will see the socket readable, which leads to a
"return true", and thus a NT_STATUS_RETRY -> same game again.

We should never get into this situation, but to me the logfiles given in bug
7518 do not reveal enough information to understand how this can happen.