Attributes: Add null check in color attribute duplicate operator

It's potentially possible that the attribute duplication could fail,
for whetever reason. There is no great reason not to be safe in
this high-level code.

source/blender/editors/geometry/geometry_attributes.cc

@@ -525,6 +525,9 @@ static int geometry_color_attribute_duplicate_exec(bContext *C, wmOperator *op)
   }
 
   CustomDataLayer *newLayer = BKE_id_attribute_duplicate(id, layer->name, op->reports);
+  if (newLayer == nullptr) {
+    return OPERATOR_CANCELLED;
+  }
 
   BKE_id_attributes_active_color_set(id, newLayer);