hlr.c: Avoid overflow of lu_operation.subscr.imsi

It appears that hlr_subscriber.imsi is 16 buffers in size:
15 chars for IMSI + 1 byte NUL.  However,  osmo_gsup_message.imsi
is 17 bytes (for whatever reason), so we cannot simply do a strpy()
as this might overflow the hlr_subscriber.imsi field!

TODO: check if weactually ever receive a too-long IMSI in GSUP and
reject that at an earlier time in the code flow.

Fixes: Coverity CID#164746

Change-Id: I9ff94e6bb0ad2ad2a7c010d3ea7dad9af0f3c048
diff --git a/src/hlr.c b/src/hlr.c
index 6310526..78a7055 100644
--- a/src/hlr.c
+++ b/src/hlr.c
@@ -164,7 +164,7 @@
 	/* check if subscriber is known at all */
 	if (!lu_op_fill_subscr(luop, g_hlr->dbc, gsup->imsi)) {
 		/* Send Error back: Subscriber Unknown in HLR */
-		strcpy(luop->subscr.imsi, gsup->imsi);
+		osmo_strlcpy(luop->subscr.imsi, gsup->imsi, sizeof(luop->subscr.imsi));
 		lu_op_tx_error(luop, GMM_CAUSE_IMSI_UNKNOWN);
 		return 0;
 	}