rPGP's integrity protection of encrypted data was not always checked

Summary

### Summary For some messages, rPGP returned incorrectly decrypted data without signaling that integrity protection was invalid. ### Details When decrypting SEIPD (Symmetrically Encrypted and Integrity Protected Data Packet), rPGP previously did not under all circumstances report the absence of valid integrity protection to callers of the library. ### Impact While the resulting invalid decryption output is not attacker controlled, its contents may be a security concern if an attacker can gain access to it. ### Attribution Discovered internally in the course of rPGP development work.