hsfz: vehicle ident string is only present in replays with non zero len. (#4757)

Author: antoniovazquezblanco

scapy/contrib/automotive/bmw/hsfz.py

@@ -65,7 +65,7 @@ class HSFZ(Packet):
         ConditionalField(
             StrFixedLenField("identification_string",
                              None, None, lambda p: p.length),
-            lambda p: p.control == 0x11)
+            lambda p: p.control == 0x11 and p.length != 0)
     ]
 
     def _hasaddrs(self):

test/contrib/automotive/bmw/hsfz.uts

@@ -106,6 +106,11 @@ assert pkt.length == 50
 assert pkt.control == 0x11
 assert b"BMW" in pkt.identification_string
 
+pkt = UDP(hex_bytes("e9811a9b000ea98f000000000011"))
+assert pkt.length == 0
+assert pkt.control == 0x11
+
+
 = Test HSFZSocket