adv_pci1710: don't access *data when insn->n == 0

adv_pci1710: don't access *data when insn->n == 0

Due to recent change to do_insnlist_ioctl() and do_insn_ioctl(), the
'data' pointer will be NULL when insn->n == 0.  Do not access *data
in this case.

Also, the INSN_WRITE handlers for AO subdevices were incorrectly storing
the data value past the end of the data array for read-back, instead of
the last data value (when insn->n > 0).

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>