Fix fuzz failure due to unexpected length in `router` target

[tnull]

In #3691 we hit a fuzz failure in CI:

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produces:

---- run_test_cases stdout ----
thread '<unnamed>' panicked at /Users/erohrer/workspace/rust-lightning/fuzz/src/router.rs:326:27:
assertion `left == right` failed:
  left: 0
 right: 546
stack backtrace:
   0: rust_begin_unwind
             at /rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library/std/src/panicking.rs:645:5
   1: core::panicking::panic_fmt
             at /rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library/core/src/panicking.rs:72:14
   2: core::panicking::assert_failed_inner
   3: core::panicking::assert_failed
             at /rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library/core/src/panicking.rs:279:5
   4: lightning_fuzz::router::do_test
   5: lightning_fuzz::router::router_test
             at ./src/router.rs:427:2
   6: router_target::run_test_cases::{{closure}}::{{closure}}
             at ./src/bin/router_target.rs:92:7
   7: std::panicking::try::do_call
             at /rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library/std/src/panicking.rs:552:40
   8: std::panicking::try
             at /rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library/std/src/panicking.rs:516:19
   9: std::panic::catch_unwind
             at /rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library/std/src/panic.rs:142:14
  10: router_target::run_test_cases::{{closure}}
             at ./src/bin/router_target.rs:91:19
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

which hits the assert_eq in https://github.com/lightningdevkit/rust-lightning/blob/main/fuzz/src/router.rs#L153

AFAIU, this was likely introduced by the changes in #3640 (cc @valentinewallace)