Object::actcol assignments from edit-mode data wasn't clamping
the index to the valid range. This caused an out of bounds read when
accessing Object::matbits.
While material indexes should typically be within the material bounds,
this isn't guaranteed. Selecting a face for example with a material
outside the range was crashing.
Add a utility function that sets the active material index to replace
existing inline checks.
Follow up to the fix for #139369.
6fab30a767