Motivation
This bug was introduced with #5377 and affects only users who'd like to share/cache/re-use `PemPrivateKey` and `PemX509Certificate` instances.
Modifications
Use `ByteBuf#writeBytes(src, readerIndex, length)` so that the src's readerIndex doesn't change and can consequently be used more than once.
Result
It's possible to share/cache/re-use `PemPrivateKey` and `PemX509Certificate` instances as long as their refCnt remains >= 1.
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