Panic in decode discovered during fuzz testing

[DavidLeeds]

https://github.com/librasn/rasn/blob/d97c44d4a58076738a2eab345101cc763ccb2178/src/ber/de.rs#L208C49-L208C55

thread fuzz_fail' panicked at .../.cargo/registry/src/index.crates.io-6f17d22bba15001f/rasn-0.24.0/src/ber/de.rs:208:55:
byte index 8 is not a char boundary; it is inside 'Ҡ' (bytes 7..9) of `+Ҡ+++'`

It appears that some additional input validation is needed to screen out unacceptable UTF-8 characters prior to indexing into the string in parse_any_generalized_time_string(). Indexing is also done here.

[XAMPPRocky]

Thank you for your issue! I don't have the capacity to write the fix, but I'm happy to review a PR 🙂