der_derive: fix derive(BitString): always encode max length #2363

Job	Run time
build (1.81.0, wasm32-unknown-unknown)	54s
build (stable, thumbv7em-none-eabi)	52s
fuzz	2m 10s
test (stable)	2m 28s
build (1.81.0, thumbv7em-none-eabi)	58s
build (stable, wasm32-unknown-unknown)	54s
test (1.81.0)	2m 33s
minimal-versions	0s
	10m 49s

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