Fix T56363: Blender crash when deleting a curve object.

Conversion code could leave object with inconsistent material data
compared to its new obdata.

Ideally, various conversion code would handle that properly, conserving
materials when possible, but for now at least ensure we get valid
result!

source/blender/editors/object/object_add.c

@@ -1903,6 +1903,9 @@ static int convert_exec(bContext *C, wmOperator *op)
 			continue;
 		}
 
+		/* Ensure new object has consistent material data with its new obdata. */
+		test_object_materials(bmain, newob, newob->data);
+
 		/* tag obdata if it was been changed */
 
 		/* If the original object is active then make this object active */