There were multiple reports with objects having many empty material
slots.
The underlying reason for this is the behavior of adding/assigning
materials [which makes room in the form of empty material slots based on
the current `active_material_index` -- which atm. can be set to
arbitrary values]. So just e.g. setting this to 100 in a fresh file and
assigning a material would create 99 empty slots.
to resolve, now clamp to the existing number of material slots.
NOTE: there is already a range function defined, but this actually only
kicks in from the animation system (so clamping would take place there),
so clamping is expected to happen in the set functions (there is also a
related comment in `RNA_property_int_set`)
Philipp Oeser