libbpf: fix another GCC8 warning for strncpy

Similar issue was fixed in cdfc7f888c2a ("libbpf: fix GCC8 warning for
strncpy") already. This one was missed. Fixing now.

Cc: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
diff --git a/tools/lib/bpf/xsk.c b/tools/lib/bpf/xsk.c
index b337402..5007b5d 100644
--- a/tools/lib/bpf/xsk.c
+++ b/tools/lib/bpf/xsk.c
@@ -517,7 +517,8 @@ int xsk_socket__create(struct xsk_socket **xsk_ptr, const char *ifname,
 		err = -errno;
 		goto out_socket;
 	}
-	strncpy(xsk->ifname, ifname, IFNAMSIZ);
+	strncpy(xsk->ifname, ifname, IFNAMSIZ - 1);
+	xsk->ifname[IFNAMSIZ - 1] = '\0';
 
 	err = xsk_set_xdp_socket_config(&xsk->config, usr_config);
 	if (err)