Fix answer function for UDS_HSFZSocket (#4776)

Author: polybassa

scapy/contrib/automotive/bmw/hsfz.py

@@ -186,7 +186,7 @@ def send(self, x):
     def recv(self, x=MTU, **kwargs):
         # type: (Optional[int], **Any) -> Optional[Packet]
         pkt = super(UDS_HSFZSocket, self).recv(x)
-        if pkt:
+        if pkt and pkt.control == 1:
             return self.outputcls(bytes(pkt.payload), **kwargs)
         else:
             return pkt