sccp: build associations on IT messages, too

Useful when you have long sessions, without the start (i.e. CC/CR) and the end
(i.e. RLSD/RLC).
Similar to 10d2e65228

Change-Id: Ifb97bd9fe88ee59f3816fce1111132b247bf46c8
Reviewed-on: https://code.wireshark.org/review/22446
Petri-Dish: Anders Broman <a.broman58@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>

epan/dissectors/packet-sccp.c

@@ -1440,6 +1440,8 @@ get_sccp_assoc(packet_info *pinfo, guint offset, sccp_decode_context_t* value)
 
     break;
   }
+  case SCCP_MSG_TYPE_IT:
+    /* fall-through */
   case SCCP_MSG_TYPE_RLC:
   {
     wmem_tree_key_t fw_key[4];