transparent: Fix bugs in `AccountPubKey::derive_pubkey_at_bip32_path` #6291

Job	Run time
Test no-Orchard on Linux	4m 29s
Slow Test Orchard on Linux	4m 9s
Test Orchard on Linux	4m 29s
Slow Test NU7 on Linux	4m 28s
Test NU7 on Linux	4m 29s
Latest build on ubuntu-latest	2m 11s
Build target wasm32-wasi	59s
Latest build on windows-latest	4m 10s
Latest build on macOS-latest	2m 42s
Check ZFuture build on Linux	26s
Test no-Orchard on macOS	4m 21s
Check ZFuture build on macOS	41s
Test Orchard on macOS	3m 33s
Check ZFuture build on Windows	2m 47s
Bitrot check	1m 37s
Test no-Orchard on Windows	4m 17s
Test Orchard on Windows	2m 44s
Test NU7 on Windows	2m 55s
Clippy (beta)	15s
Clippy (MSRV)	1m 22s
Code coverage	4m 17s
Intra-doc links	2m 16s
Rustfmt	17s
protobuf consistency	1m 9s
UUID validity	4s
Required status checks have passed	1s
	1h 5m 8s

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