test: fix thinko in gossipd/test/run-bench-find_route.c

Reported-by: @cdecker
Fixes: #2440
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

gossipd/test/run-bench-find_route.c

@@ -162,7 +162,7 @@ static struct pubkey nodeid(size_t n)
 	struct secret s;
 
 	memset(&s, 0xFF, sizeof(s));
-	memcpy(&s, &n, sizeof(s) / sizeof(n));
+	memcpy(&s, &n, sizeof(n));
 	pubkey_from_secret(&s, &id);
 	return id;
 }