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>

net/9p/client.c

@@ -541,6 +541,8 @@ static int p9_check_errors(struct p9_client *c, struct p9_req_t *req)
 		kfree(ename);
 	} else {
 		err = p9pdu_readf(&req->rc, c->proto_version, "d", &ecode);
+		if (err)
+			goto out_err;
 		err = -ecode;
 
 		p9_debug(P9_DEBUG_9P, "<<< RLERROR (%d)\n", -ecode);