MDEV-19581 Valgrind error with WolfSSL and encrypted binlog

WolfSSL can read  memory out of bounds in EVP_CipherUpdate()
in decrypt/NOPAD mode, when the input length is not multiple of AES block
size.

The workaround ensures that input will have some padding at the end
by having slightly larger allocated buffer, or padding the structures
with 16 more bytes.