nat: Remember the original dest local reference in the parsed struct

In case of the RLSD coming from the MSC we are patching the address
in-situ but for local calls set con = NULL. We then answered the RLSD
with the wrong reference and the MSC kept on trying.
diff --git a/openbsc/src/osmo-bsc_nat/bsc_filter.c b/openbsc/src/osmo-bsc_nat/bsc_filter.c
index bbbba69..6a9e99f 100644
--- a/openbsc/src/osmo-bsc_nat/bsc_filter.c
+++ b/openbsc/src/osmo-bsc_nat/bsc_filter.c
@@ -120,6 +120,8 @@
 		parsed->sccp_type = sccp_determine_msg_type(msg);
 		parsed->src_local_ref = result.source_local_reference;
 		parsed->dest_local_ref = result.destination_local_reference;
+		if (parsed->dest_local_ref)
+			parsed->original_dest_ref = *parsed->dest_local_ref;
 		parsed->called_ssn = result.called.ssn;
 		parsed->calling_ssn = result.calling.ssn;