hnbgw_hnbap.c: Clean up after encoding a UE register accept

Need to free the ASN.1 structs after using them to encode the UE
register accept message.
diff --git a/src/hnbgw_hnbap.c b/src/hnbgw_hnbap.c
index fb59ad2..0b8e3e4 100644
--- a/src/hnbgw_hnbap.c
+++ b/src/hnbgw_hnbap.c
@@ -104,6 +104,10 @@
 						Criticality_reject,
 						&asn_DEF_UERegisterAccept,
 						&accept_out);
+
+	ASN_STRUCT_FREE_CONTENTS_ONLY(asn_DEF_OCTET_STRING, &accept.uE_Identity.choice.iMSI);
+	ASN_STRUCT_FREE_CONTENTS_ONLY(asn_DEF_UERegisterAccept, &accept_out);
+
 	return hnbgw_hnbap_tx(ue->hnb, msg);
 }