dnp: plug a memory leak.

If we're throwing away the data, *throw away the data* - free it, as
we're not using it as the backing data for a tvbuff.


(cherry picked from commit 618661b22e)

epan/dissectors/packet-dnp.c

@@ -3455,6 +3455,7 @@ dissect_dnp3_message(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, void*
     else
     {
       /* CRC error - throw away the data. */
+      g_free(al_buffer);
       next_tvb = NULL;
     }
   }