85bf6814c7
Similar to 64e955f522 (but extended a bit).
Fix by preventing access of non-existent (or out of bounds)
`TexPaintSlot`.
In the future, we should probably even prevent it further by using
`RNA_def_property_int_funcs` (but that is for a later commit).
Pull Request: https://projects.blender.org/blender/blender/pulls/116345