libsa: smbios: Reject a 64-bit entry point with revision 0

According to the specification, such an entry point may have different
data in bytes at offsets 0x0c to 0x17 (included). In such a case,
interpreting them as the Structure Table Maximum Size and Address fields
could have catastrophic consequences.

Reviewed by: imp, markj
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49286