libc: fix size check in select()

select() previously returned EINVAL only when nfds > FD_SETSIZE+1. The
right test is nfds > FD_SETSIZE, i.e., for nfds = FD_SETSIZE+1 this is also
an error.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>