Skip empty lines on incoming routes data

If we receive no routes from server or if, for some reason, we receive
some empty lines, we should skip them instead of crashing.

Fixes on of the problems in #147.

sshuttle/client.py

@@ -489,6 +489,7 @@ def _main(tcp_listener, udp_listener, fw, ssh_cmd, remotename,
     def onroutes(routestr):
         if auto_nets:
             for line in routestr.strip().split(b'\n'):
+                if not line: break
                 (family, ip, width) = line.split(b',', 2)
                 family = int(family)
                 width = int(width)