libcli/security:sddl_decode message offset safety latch

libcli/security:sddl_decode message offset safety latch

the message offset is largely calculated using the differences
between pointers in many places scattered throughout the code.

If we got one of these wrong, we could easily have a SIZE_MAX-ish
offset, which would be unfortunate if we came decided to display
the offset using spaces.

We can sanely limit the offset to the length of the SDDL.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>