s526: don't access *data when insn->n == 0

s526: 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 code for pulse-train generation should only access
data[1] if insn->n >= 2.

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