x.crypto.chacha20: add a check counter overflow to set_counter

[blackshirt]

This small PR contains two small fix,

1. The first part was adding check into .set_counter routine, especially in standard variant where its only allowing 32bit counter values.
2. The second fixs the warning when run the test of chacha_64bitctr_test with -cstrict options, would lead into error: integer constant is so large that it is unsigned [-Werror] because counter was defined as int before changes.

Thanks

[huly-for-github]

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