const-oid: fix encoder `base128_len` calculation #2392

Job	Run time
test (1.81.0)	2m 36s
build (1.81.0, thumbv7em-none-eabi)	53s
fuzz	2m 18s
build (1.81.0, wasm32-unknown-unknown)	59s
test (stable)	2m 23s
build (stable, wasm32-unknown-unknown)	52s
build (stable, thumbv7em-none-eabi)	51s
minimal-versions	-1s
	10m 51s

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