nat: Keep track of both sides of the connection

On a CC message we will need to remeber where the source local
reference of the network belonged so we can properly identify
the connection when receiving UDT messages.
diff --git a/openbsc/src/nat/bsc_nat.c b/openbsc/src/nat/bsc_nat.c
index 65fc10b..cafca83 100644
--- a/openbsc/src/nat/bsc_nat.c
+++ b/openbsc/src/nat/bsc_nat.c
@@ -191,7 +191,11 @@
 		case SCCP_MSG_TYPE_RLSD:
 		case SCCP_MSG_TYPE_CREF:
 		case SCCP_MSG_TYPE_DT1:
+			bsc = patch_sccp_src_ref_to_bsc(msg, parsed, nat);
+			break;
 		case SCCP_MSG_TYPE_CC:
+			if (update_sccp_src_ref(bsc, msg, parsed) != 0)
+				goto exit;
 			bsc = patch_sccp_src_ref_to_bsc(msg, parsed, nat);
 			break;
 		case SCCP_MSG_TYPE_RLC: