libelf: Correct overflow check in note_xlate.

We want to make sure the note_len doesn't overflow and becomes shorter
than the note header. But the namesz and descsz checks got the note header
size wrong). Replace the wrong constant (8) with a sizeof cvt_Nhdr (12).

https://sourceware.org/bugzilla/show_bug.cgi?id=24084



Signed-off-by: Mark Wielaard <mark@klomp.org>