base64ct: Don't fail with `InvalidLength` when reading nothing at end… #2322

Job	Run time
build (1.81.0, thumbv7em-none-eabi)	57s
build (stable, wasm32-unknown-unknown)	45s
fuzz	2m 14s
test (1.81.0)	2m 35s
build (1.81.0, wasm32-unknown-unknown)	59s
build (stable, thumbv7em-none-eabi)	44s
test (stable)	2m 20s
minimal-versions	0s
	10m 34s

Provide feedback