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

Fixes: coverity scan CID 163918
Change-Id: I4b2760b006a0707928530b4390c6997b79b02981
diff --git a/openbsc/src/gprs/gprs_subscriber.c b/openbsc/src/gprs/gprs_subscriber.c
index 2042ec6..1bb5141 100644
--- a/openbsc/src/gprs/gprs_subscriber.c
+++ b/openbsc/src/gprs/gprs_subscriber.c
@@ -157,7 +157,7 @@
 		gsub = gprs_subscr_alloc();
 		if (!gsub)
 			return NULL;
-		strncpy(gsub->imsi, imsi, sizeof(gsub->imsi));
+		osmo_strlcpy(gsub->imsi, imsi, sizeof(gsub->imsi));
 	}
 
 	if (!gsub->sgsn_data)