gprs_ns2: Don't return an empty string in case of unknown LL

Change-Id: Id2c689ab83ada18a5cb6d70ded31aacaa701f99c
diff --git a/src/gb/gprs_ns2.c b/src/gb/gprs_ns2.c
index 7fc737b..9bf1d7f 100644
--- a/src/gb/gprs_ns2.c
+++ b/src/gb/gprs_ns2.c
@@ -265,7 +265,7 @@
 			 gprs_ns2_fr_nsvc_dlci(nsvc));
 		break;
 	default:
-		buf[0] = '\0';
+		snprintf(buf, buf_len, "unknown)");
 		break;
 	}