ENT-13591: Fixed buffer overflow in cf-secret when using multiple keys of different sizes

[larsewi]

$ openssl genrsa -out small.priv 2048
$ openssl rsa -in small.priv -RSAPublicKey_out -out small.pub
writing RSA key
$ openssl genrsa -out large.priv 4096
$ openssl rsa -in large.priv -RSAPublicKey_out -out large.pub
writing RSA key
$ echo "Secret Data" > secret.txt
$ cf-secret encrypt -k small.pub,large.pub -o secret.enc secret.txt
free(): invalid next size (normal)
Aborted (core dumped)

Ticket: ENT-13591
Changelog: Title
Signed-off-by: Lars Erik Wik lars.erik.wik@northern.tech

Backported to #6000

[larsewi]