Panic on incorrect date input to `simple_asn1`

Summary

Version 0.6.0 of the `simple_asn1` crate panics on certain malformed inputs to its parsing functions, including `from_der` and `der_decode`. Because this crate is frequently used with inputs from the network, this should be considered a security vulnerability. The issue occurs when parsing the old ASN.1 "UTCTime" time format. If an attacker provides a UTCTime where the first character is ASCII but the second character is above 0x7f, a string slice operation in the `from_der_` function will try to slice into the middle of a UTF-8 character, and cause a panic. This error was introduced in commit [`d7d39d709577710e9dc8`](https://github.com/acw/simple_asn1/commit/d7d39d709577710e9dc8833ee57d200eef366db8), which updated `simple_asn1` to use `time` instead of `chrono` because of [`RUSTSEC-2020-159`](https://rustsec.org/advisories/RUSTSEC-2020-0159). Versions of `simple_asn1` before 0.6.0 are not affected by this issue. The [patch](https://github.com/acw/simple_asn1/pull/28) was applied in `simple_asn1` version 0.6.1.