[AD] fixmul() not behaving as supposed to

[Grzegorz Adam Hankiewicz <gradha@xxxxxxxxxx>]

Using "fixtof(fixmul(itofix(2), itofix(32000));" on my machine
doesn't seem to set errno nor return the clamped value of
the multiplication.  I'm using the C inline version found at
include/inline/fmaths.inl, line 115.  No idea what to make out from
the code.