Fix bug for nested struct array

[zcmmwbd]

Fix the bug so that the following code can run normally

    s = borsh.schema({
        "struct_array": borsh.types.fixed_array(borsh.types.struct({
                "nested_struct_field1": borsh.types.u8
            }), 3)
    })
    arr = [{ "nested_struct_field1": 1 }, { "nested_struct_field1": 2 }, { "nested_struct_field1": 3 }]
    data = borsh.serialize(s, {
        "struct_array": [ borsh.types.struct(i) for i in arr ]
    })
    print(borsh.deserialize(s, data))