The issue was that some `material_index` were `-1`. This meant that
the rendering code was accessing out-of-bounds memory to read
from the material pool.
The fix adds an assert to the `gpencil_material_resources_get` and
clamps the `material_index` to >= 0 to ensure that the material is
valid. It also changes the default read value for the material indices
in `create_curves_outline` and `retrieve_visible_strokes` to be 0
instead of -1.
