transparent: Fix bugs in AccountPubKey::derive_pubkey_at_bip32_path

[str4d]

• A typo in a panic guard condition instead exposed the panic.
• The match logic wasn't correctly handling the first element of the path.

[str4d]

Force-pushed to fix clippy lint.

[str4d]

Force-pushed to extend the test.

[nuttycom]

utACK 06f3eb7

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 53.01%. Comparing base (a1e4e7a) to head (06f3eb7).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
zcash_transparent/src/keys.rs	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1667      +/-   ##
==========================================
+ Coverage   52.94%   53.01%   +0.06%     
==========================================
  Files         175      175              
  Lines       21072    21073       +1     
==========================================
+ Hits        11157    11171      +14     
+ Misses       9915     9902      -13     

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