compat_freebsd4: Check for errors from subyte() in freebsd4_uname()

This is in preparation for adding a __result_use_check annotation to
copyin() and related functions.

Reviewed by: imp, kib, jhb
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D43099

(cherry picked from commit 4f35450ce52a7b141e7ae8d37fa257b5f8971dda)