[sccp] Set the data_len when parsing UDT messages as well

For UDT the msgb_l3len can be safely used but it is better
to set it to data_len for everyone..
diff --git a/openbsc/src/sccp/sccp.c b/openbsc/src/sccp/sccp.c
index 7630b1c..a199266 100644
--- a/openbsc/src/sccp/sccp.c
+++ b/openbsc/src/sccp/sccp.c
@@ -459,6 +459,7 @@
 
 
 	msgb->l3h = &udt->data[udt->variable_data];
+	result->data_len = msgb_l3len(msgb);
 
 	if (msgb_l3len(msgb) !=  msgb->l3h[-1]) {
 		DEBUGP(DSCCP, "msgb is truncated is: %u should: %u\n",