spnego: don't synthesise a GSSAPI blob from an empty token

scapy/asn1fields.py

@@ -1085,4 +1085,6 @@ def __init__(self,
     def m2i(self, pkt, s):  # type: ignore
         # type: (ASN1_Packet, bytes) -> Tuple[ASN1_Packet, bytes]
         val = super(ASN1F_STRING_ENCAPS, self).m2i(pkt, s)
+        if not val[0].val:
+            return val[0], val[1]  # type: ignore
         return self.cls(val[0].val, _underlayer=pkt), val[1]

scapy/layers/spnego.py

@@ -143,6 +143,8 @@ def i2m(self, pkt, x):
 
     def m2i(self, pkt, s):
         dat, r = super(_SPNEGO_Token_Field, self).m2i(pkt, s)
+        if not dat.val:
+            return dat, r
         types = None
         if isinstance(pkt.underlayer, SPNEGO_negTokenInit):
             types = pkt.underlayer.mechTypes

test/scapy/layers/spnego.uts

@@ -437,3 +437,16 @@ try:
     assert False, "Should have failed !"
 except ValueError:
     pass
+
++ SPNEGO packets
+
+= SPNEGO_Token - build and dissect empty payloads
+
+pkt = SPNEGO_Token()
+assert raw(pkt) == b"\x04\x00"
+assert raw(SPNEGO_Token(raw(pkt))) == raw(pkt)
+assert not SPNEGO_Token(raw(pkt)).value
+
+pkt = SPNEGO_MechListMIC()
+assert raw(pkt) == b"\x04\x00"
+assert raw(SPNEGO_MechListMIC(raw(pkt))) == raw(pkt)