ReadPendingInput plays unsafe byte/char games with pending data

[miniksa]

Ported from MSFT: 18047766.

I fixed an immediate crash bug in MSFT: 17987369 that affected this area, but realized at the time that it's really bad that the ReadPendingInput function in the console host is playing games with a void pointer and mix-and-match of byte versus character counting.

This represents investigating what we can do to stop mix/matching the char and byte counts in ReadPendingInput.

@adiviness might be working on something like this with his changes to input to support emoji...

[adiviness]

Yup, I'm going to have to fix this up as well.