read_ns_sys_file(): don't overrun buf

This is a classical off-by-one error. If sizeof(buf) is 512,
the last element is buf[511] but not buf[512].

Note that if read() returns 0, we return 0 but buf stays
uninitialized.

Reported by Coverity, CID 114623.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>