9p/net: fix missing error check in p9_check_errors

Link: https://lkml.kernel.org/r/99338965-d36c-886e-cd0e-1d8fff2b4746@gmail.com


Reported-by:  <syzbot+06472778c97ed94af66d@syzkaller.appspotmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>