mss1: improve check if decoded pivot is invalid

The pivot has to lie between 0 and base.
Check of ==base is insufficient.
Thus replace it by a proper check.

Fixes out of array write.

Fixes bug #1531.

Found-by: Piotr Bandurski <ami_stuff@o2.pl>
Signed-off-by: Paul B Mahol <onemda@gmail.com>
