p + FAST_SIZE > buffer + length

In this sentence p + FAST_SIZE may be bigger than (1<<32),
and we will be in trouble.

$ gdb -c coredump test/zdtm/static/write_read01

(gdb) p p
$3 = (uint8_t *) 0xffffa89e
(gdb) p buffer
$4 = (uint8_t *) 0xfff06780
(gdb) p length
$5 = 1000000
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>