s_mp_grow
mp_err s_mp_grow(mp_int *mp, mp_size min); /* increase allocated size */
if ((res = s_mp_grow(mp, min)) != MP_OKAY)
if((res = s_mp_grow(mp, ALLOC(mp) + 1)) != MP_OKAY)
MP_CHECKOK( s_mp_grow(sqr, ix) );