[libfko] Bug fix to not attempt SPA packet decryption with GnuPG without
an fko object with encryption_mode set to FKO_ENC_MODE_ASYMMETRIC. This
bug was caught with valgrind validation against the perl FKO extension
together with the set of SPA fuzzing packets in
test/fuzzing/fuzzing_spa_packets. Note that this bug cannot be
triggered via fwknopd because additional checks are made within fwknopd
itself to force FKO_ENC_MODE_ASYMMETRIC whenever an access.conf stanza
contains GPG key information. This fix strengthens libfko itself to
independently require that the usage of fko objects without GPG key
information does not result in attempted GPG decryption operations. Hence
this fix applies mostly to third party usage of libfko - i.e. stock
installations of fwknopd are not affected. As always, it is recommended to
use HMAC authenticated encryption whenever possible even for GPG modes since
this also provides a work around even for libfko prior to this fix.