Modified to meet PR standards. See PR conversation

--HG--
branch : issue_5141

scapy/contrib/cdp.py

@@ -289,17 +289,17 @@ class CDPMsg(CDPMsgGeneric):
 
 class _CDPChecksum:
     def _check_len(self, pkt):
-        """Check for odd packet length and pad according to Cisco spec"""
+        """Check for odd packet length and pad according to Cisco spec.
+        This padding is only used for checksum computation.  The original
+        packet should not be altered."""
         if len(pkt) % 2:
             last_chr = pkt[-1]
-            if ord(last_chr) <=0x80:
-                ck_p = pkt[:-1] + '\x00' + last_chr
+            if last_chr <= '\x80':
+                return pkt[:-1] + '\x00' + last_chr
             else:
-                ck_p = pkt[:-1] + '\xff' + chr(ord(last_chr) - 1)
+                return pkt[:-1] + '\xff' + chr(ord(last_chr) - 1)
         else:
-            ck_p = pkt
-
-        return ck_p
+            return pkt
 
     def post_build(self, pkt, pay):
         p = pkt + pay