[nat/bsc] Fix memory leak of IPA messages...

* The read_msg method is allocating the msgb and we will
  need to free it once we are done with it.
diff --git a/openbsc/src/nat/bsc_nat.c b/openbsc/src/nat/bsc_nat.c
index e4db47f..8e31188 100644
--- a/openbsc/src/nat/bsc_nat.c
+++ b/openbsc/src/nat/bsc_nat.c
@@ -401,6 +401,7 @@
 	else if (hh->proto == IPAC_PROTO_SCCP)
 		forward_sccp_to_bts(msg);
 
+	talloc_free(msg);
 	return 0;
 }
 
@@ -567,6 +568,7 @@
 	/* FIXME: Currently no PONG is sent to the BSC */
 	/* FIXME: Currently no ID ACK is sent to the BSC */
 	forward_sccp_to_msc(bfd, msg);
+	talloc_free(msg);
 
 	return 0;
 }