libcli: continue to read from the socket even if the size is 0

libcli: continue to read from the socket even if the size is 0

This is an issue found by Codenomicon, with a malicious packet with 0
bytes UDP payload we will continiously be looping trying to react from
the socket event and continiously do nothing as we will bail out
thinking that we had a memory allocation error.

Original fix comes from Volker Lendecke <vl@samba.org>

Signed-off-by: Matthieu Patou <mat@matws.net>
Reviewed-by: Volker Lendecke <vl@samba.org>
Autobuild-User(master): Volker Lendecke <vl@samba.org>
Autobuild-Date(master): Fri Sep 20 04:46:47 CEST 2013 on sn-devel-104