[libfko] bug fix to check b64_decode() return value

[libfko] bug fix to check b64_decode() return value

Bug fix to check b64_decode() return value to ensure that
non-base64 encoded data is never used.  Even though other validation
routines checked decoded results, it is important to discard invalid
data as early as possible.  Note too that such invalid data would only
be provided to b64_decode() after proper decryption, so the client must
provide authentic SPA data.