nat: Always initialize the out pointer...

Always initialize the pointer to a invalid value in case
we encounter a parsing error or such.
diff --git a/openbsc/src/nat/bsc_nat_utils.c b/openbsc/src/nat/bsc_nat_utils.c
index 297d45e..0815a4a 100644
--- a/openbsc/src/nat/bsc_nat_utils.c
+++ b/openbsc/src/nat/bsc_nat_utils.c
@@ -112,6 +112,8 @@
 	struct tlv_parsed tp;
 	int i = 0;
 
+	*lac_out = -1;
+
 	if (!msg->l3h || msgb_l3len(msg) < 3) {
 		LOGP(DNAT, LOGL_ERROR, "Paging message is too short.\n");
 		return NULL;