s4:ntvfs: Avoid signed integer overflow

s4:ntvfs: Avoid signed integer overflow

Signed integer overflow is undefined behaviour.

ubsan reports errors similar to the following:
signed integer overflow: 9223372036854775807 + 2147483682 cannot be represented in type 'long int'

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>