Ensure discover() returns devices and not None

There's a small race condition in the discover() method. If recv() returns
before the socket timeout is reached, but we hit the top of the loop after
the timeout is reached, we'll return None rather than the set of devices.

broadlink/__init__.py

@@ -126,6 +126,7 @@ def discover(timeout=None, local_ip_address=None):
       mac = responsepacket[0x3a:0x40]
       dev = gendevice(devtype, host, mac)
       devices.append(dev)
+    return devices
 
 
 class device: