utils: check snprintf return value

src/lxc/utils.c

@@ -1097,7 +1097,9 @@ int __safe_mount_beneath_at(int beneath_fd, const char *src, const char *dst, co
 	target_fd = openat2(beneath_fd, dst, &how, sizeof(how));
 	if (target_fd < 0)
 		return -errno;
-	snprintf(tgt_buf, sizeof(tgt_buf), "/proc/self/fd/%d", target_fd);
+	ret = snprintf(tgt_buf, sizeof(tgt_buf), "/proc/self/fd/%d", target_fd);
+	if (ret < 0 || ret >= sizeof(tgt_buf))
+		return -EIO;
 
 	if (!is_empty_string(src_buf))
 		ret = mount(src_buf, tgt_buf, fstype, flags, data);