Fix #105577: Python MeshPolygon API allows negative material indices

This was lost in the refactor to store material indices in a generic attribute.
The attribute API still allows this, but that will be handled separately
since it's a more complex task. The existing API that already clamped
input values should still do that.

source/blender/makesrna/intern/rna_mesh.c

@@ -698,7 +698,7 @@ static void rna_MeshPolygon_material_index_set(PointerRNA *ptr, int value)
   Mesh *mesh = rna_mesh(ptr);
   int *material_indices = BKE_mesh_material_indices_for_write(mesh);
   const int index = rna_MeshPolygon_index_get(ptr);
-  material_indices[index] = value;
+  material_indices[index] = max_ii(0, value);
 }
 
 static void rna_MeshPolygon_center_get(PointerRNA *ptr, float *values)