more size checks needed in f_hmi.c, f_hmp.c, and possibly f_xmidi.c

[sezero]

To fix bug #175 we added multiple size checks, but f_hmi.c, f_hmp.c,
and possibly f_xmidi.c will need similar care. I added some initial,
rudimentary checks in commits 05356fc and 89b565d (and 8d995c6
and a889344), but they definitely need improving.

[psi29a]

Thanks for creating these checks. :)

[sezero]

That's no problem. But as I said those sources need some love.

[sezero]

This still needs to be done.