This commit fixes a double free condition discovered through the new
python SPA payload fuzzer. This bug could be triggered in fwknopd with
a malicious SPA payload but only when GnuPG is used. When Rijndael is
used for SPA packet encryption, this bug cannot be triggered due to an
length/format check towards the end of _rijndael_decrypt(). It should
be noted that only a person in possession of the correct encryption and
authentication GnuPG keys could trigger this bug.