RNP 0.18.0 Vulnerable PKESK session keys

Summary

In RNP version 0.18.0 a refactoring regression causes the symmetric session key used for Public-Key Encrypted Session Key (PKESK) packets to be left uninitialized except for zeroing, resulting in it always being an all-zero byte array. Any data encrypted using public-key encryption in this release can be decrypted trivially by supplying an all-zero session key, fully compromising confidentiality. The vulnerability affects only public key encryption (PKESK packets).  Passphrase-based encryption (SKESK packets) is not affected. Root cause: Vulnerable session key buffer used in PKESK packet generation. The defect was introduced in commit `7bd9a8dc356aae756b40755be76d36205b6b161a` where initialization logic inside `encrypted_build_skesk()` only randomized the key for the SKESK path and omitted it for the PKESK path.