crypto-api: always allocate memory when serializing iovec_t

The AEAD iov interface falls back to serializing the input buffers if the low-level cipher doesn't support scatter/gather encryption. However, there was a bug in the functions used for the serialization, which causes memory leaks under a certain condition (i.e. the number of input buffers is 1).

This patch makes the logic of the functions simpler, by removing a micro-optimization that tries to minimize the number of calls to malloc/free.

The original problem was reported by Marius Steffen in: https://bugzilla.samba.org/show_bug.cgi?id=14399 and the cause was investigated by Alexander Haase in: !1277 (closed)

Fixes #1017 (closed).

Checklist

• Commits have Signed-off-by: with name/author being identical to the commit author
• Code modified for feature
• Test suite updated with functionality tests
• Test suite updated with negative tests
• Documentation updated / NEWS entry present (for non-trivial changes)
• CI timeout is 2h or higher (see Settings/CICD/General pipelines/Timeout)

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTION.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code