```
SELECT
    CAST(([2, 1, 1023], ['', '']), 'Map(UInt8, String)') AS map,
    map[10]
```

Triggers `Assertion 'n < size()' failed` in `DB::ColumnString::operator[](unsigned long)`
https://clickhouse-test-reports.s3.yandex.net/18481/64c4ade5b0e0c9a13d6c2475431daf431fc97554/fuzzer/fuzzer.log

Introduced here: #17829
First found here: #18481
Additions additions [[maybe_unused]] = Additions())
    {
        return arguments[0].column;
    }
};
/** Conversion from FixedString to String.
  * Cutting sequences of zero bytes from end of strings.
  */
template <typename Name>
struct ConvertImpl<DataTypeFixedString, DataTypeString, Name, ConvertDefaultBehaviorTag>
{
    static ColumnPtr execute(const ColumnsWithTypeAndName & arguments, const DataTypePtr &, size_t )
    {
        if (const ColumnFixedString * col_from = checkAndGetColumn<ColumnFixedString>(arguments[0].column.get()))
        {
            auto col_to = ColumnString::create();
            const ColumnFixedString::Chars & data_from = col_from->getChars();
            ColumnString::Chars & data_to = col_to->getChars();
            ColumnString::Offsets & offsets_to = col_to->getOffsets();
            size_t size = col_from->size();
            size_t n = col_from->getN();
            data_to.resize(size * (n + 1)); 
            offsets_to.resize(size);
            size_t offset_from = 0;
            size_t offset_to = 0;
            for (size_t i = 0; i < size; ++i)
            {