smbios: Unmap memory on error on identify

While here, de-indent most of the code by simply bailing out if 'addr'
is still 0 after the various detection methods have been tried.

Reviewed by: emaste, imp
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49180

(cherry picked from commit 67d510f0c07afd89e51e337e5abec47f4483ecd9)