Tests: Fix deserialization of reject messages

Assume that reject messages for blocks or transactions due to reason
REJECT_MALFORMED will not include the hash of the block or tx being rejected.

qa/rpc-tests/test_framework/mininode.py

@@ -983,6 +983,7 @@ class msg_headers(object):
 
 class msg_reject(object):
     command = b"reject"
+    REJECT_MALFORMED = 1
 
     def __init__(self):
         self.message = b""
@@ -994,14 +995,16 @@ class msg_reject(object):
         self.message = deser_string(f)
         self.code = struct.unpack("<B", f.read(1))[0]
         self.reason = deser_string(f)
-        if (self.message == "block" or self.message == "tx"):
+        if (self.code != self.REJECT_MALFORMED and
+                (self.message == b"block" or self.message == b"tx")):
             self.data = deser_uint256(f)
 
     def serialize(self):
         r = ser_string(self.message)
         r += struct.pack("<B", self.code)
         r += ser_string(self.reason)
-        if (self.message == "block" or self.message == "tx"):
+        if (self.code != self.REJECT_MALFORMED and
+                (self.message == b"block" or self.message == b"tx")):
             r += ser_uint256(self.data)
         return r