Harden the current checks in place, I doubt these will ever hit (you can
prove easily by reading the current source code this cannot happen) but
just in case a new Go version does something weird or something else
goes catastrophicly wrong, this should add an extra defense-in-depth
layer.
`n != aeadKeySize` will panic a nil error, don't think it's needed to
add more logic to this, a nil error is enough to indicate that that
condition failed (given the other condition is `err != nil`).
Also move constant integers to being `const`, this helps reducing the
amount of instructions being done for the extra check.
Gusted
967603abcc