bsc_/gprs_subscriber: fix: use osmo_strlcpy() to safely copy IMSI

Fixes: coverity scan CID 163918
Change-Id: I4b2760b006a0707928530b4390c6997b79b02981
diff --git a/openbsc/src/libbsc/bsc_subscriber.c b/openbsc/src/libbsc/bsc_subscriber.c
index a5e40cd..73e61e8 100644
--- a/openbsc/src/libbsc/bsc_subscriber.c
+++ b/openbsc/src/libbsc/bsc_subscriber.c
@@ -80,7 +80,7 @@
 {
 	if (!bsub)
 		return;
-	strncpy(bsub->imsi, imsi, sizeof(bsub->imsi));
+	osmo_strlcpy(bsub->imsi, imsi, sizeof(bsub->imsi));
 }
 
 struct bsc_subscr *bsc_subscr_find_or_create_by_imsi(struct llist_head *list,