Fix possible infinite loop on pipe/sock_drain()

Fix possible infinite loop on pipe/sock_drain()

If the returned value of read/recv is 0 (meaning EOF), we'll end up in an
infinite loop (active wait) until something is written on the pipe which is
not really what we want here especially because those functions are called
from the main thread.

Signed-off-by: David Goulet <dgoulet@ev0ke.net>