Fix custody subnet count encoding #17248

Job	Run time
Check for 'skip-ci' label	1s
check-code	8m 15s
check-msrv	4m 44s
target-branch-check	1s
beacon-chain-tests	21m 3s
cargo-udeps	6m 42s
cli-check	15m 42s
basic-simulator-ubuntu	21m 15s
compile-with-beta-compiler	14m 7s
debug-tests-ubuntu	10m 22s
ef-tests-ubuntu	12m 52s
execution-engine-integration-ubuntu	10m 54s
fallback-simulator-ubuntu	19m 14s
network-tests	19m 17s
op-pool-tests	21m 32s
release-tests-ubuntu	16m 47s
slasher-tests	3m 21s
state-transition-vectors-ubuntu	9m 24s
test-suite-success	7s
	3h 35m 40s

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