Avoid empty hwaddr

It has been observed, at least once, for an empty hwaddr to appear.
Take measures to prevent this from occurring and making into the
discovery info.

confluent_server/confluent/discovery/core.py

@@ -510,7 +510,7 @@ def _recheck_single_unknown_info(configmanager, info):
 
 
 def safe_detected(info):
-    if 'hwaddr' not in info:
+    if 'hwaddr' not in info or not info['hwaddr']:
         return
     if info['hwaddr'] in runningevals:
         # Do not evaluate the same mac multiple times at once

confluent_server/confluent/discovery/protocols/pxe.py

@@ -92,7 +92,7 @@ def snoop(handler):
         rq = bytearray(rq)
         if rq[0] == 1:  # Boot request
             addrlen = rq[2]
-            if addrlen > 16:  # max address size in bootp is 16 bytes
+            if addrlen > 16 or addrlen == 0:
                 continue
             netaddr = rq[28:28+addrlen]
             netaddr = ':'.join(['{0:02x}'.format(x) for x in netaddr])