wireguard: T7246: verify Base64 encoded 32byte boundary on keys

Not 31 bytes or 33 bytes, but exactly 32. This matters, because 32 does not
divide evenly by .75, so there's a padding character and the penultimate
character does not include the whole base64 alphabet.

Extend the base64 validator with an optional argument to define the length
to match of the decrypted Base64 encoded string.

Source: https://lists.zx2c4.com/pipermail/wireguard/2020-December/006222.html