build-id: fix minor quirks

build-id: fix minor quirks

The initial size check in xen_build_id_check() came too late (after the
first access to the structure), but was mostly redundant with checks
done in all callers; convert it to a properly placed ASSERT(). The
"mostly" part being addressed too: xen_build_init() was off by one.

And then there was a stray semicolon.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>