Fix memleak in 'iov_store_grow'

This patch is related to issue #1017 (closed) and fixes a bug in iov_store_grow() to avoid a memory leak for use with Samba and AES-CCM cipher.

Under certain conditions memory is not freed after use, as the allocated flag is unset. In use with Samba, this can lead to a DoS attack, as copying a sufficiently large amount of data would allocate the whole memory of the server and letting the process die. As far as I could test, this patch allows the memory to be freed up again, having a stable memory consumption when using gnutls_aead_cipher_decryptv2().

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