Fix a multi-thread deadlock

Fix a multi-thread deadlock

This fixes a deadlock which was seen in-the-wild with wine.

It could happen that two threads tried to read from the socket at the same time
and one of the thread got stuck inside of poll()/select().

The fix works by making sure that the writing thread doesn't steal the reading
thread's reply.

Debugged-by: Erich Hoover <ehoover@mines.edu>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=54671
Signed-off-by: Uli Schlachter <psychon@znc.in>