base64ct: Don't fail with `InvalidLength` when reading nothing at end of data #2321

Job	Run time
build (1.81.0, thumbv7em-none-eabi)	51s
build (1.81.0, wasm32-unknown-unknown)	50s
build (stable, thumbv7em-none-eabi)	45s
build (stable, wasm32-unknown-unknown)	47s
minimal-versions	0s
fuzz	2m 10s
test (1.81.0)	2m 28s
test (stable)	2m 44s
	10m 35s

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