Resolve decryption failure when message has no version header

Summary: BUG 357462

Test Plan:
1. Copy a message without the version header into KGpg (and no blank line/whitespace between the begin PGP message header and ciphertext.)
2. Decrypt message
3. Decryption succeed.

Reviewers: dakon

Reviewed By: dakon

Subscribers: kde-utils-devel

Differential Revision: https://phabricator.kde.org/D12769