DOCSIS: Prevent infinite loop from unknown FCParm.

concatlen was not updated if FCParm was unknown, leading to an infinite loop.

Bug: 13797
Change-Id: I1b64d757a369183a711f01b0b5cd1ba7aa0787bc
Reviewed-on: https://code.wireshark.org/review/22120
Reviewed-by: Michael Mann <mmann78@netscape.net>
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>

plugins/docsis/packet-docsis.c

@@ -825,6 +825,10 @@ dissect_docsis (tvbuff_t * tvb, packet_info * pinfo, proto_tree * tree, void* da
           concatpos = 0;
           break;
         }
+        default:
+            /* Unknown parameter, stop dissection */
+          concatlen = 0;
+          break;
       } /* switch fcparm */
       break;
     }