tls: remove harmful unnecessary bounds checking

The EncIn, EncOut, ClearIn & ClearOut functions are victims of some code
copy + pasting. A common line copied to all of them is:

`if (off >= buffer_length) { ...`

448e0f43 corrected ClearIn's check from `>=` to `>`, but left the others
unchanged (with an incorrect bounds check). However, if you look down at
the next very next bounds check you'll see:

`if (off + len > buffer_length) { ...`

So the check is actually obviated by the next line, and should be
removed.

This fixes an issue where writing a zero-length buffer to an encrypted
pair's *encrypted* stream you would get a crash.