Fix bug 613: psqrt was incorrect for small numbers
(transplanted from 34f92f66abdbfb9f95fa5956cbee332b849e418e)

--HG--
branch : 3.1
extra : transplant_source : 4%F9/f%AB%DB%FB%9F%95%FAYV%CB%EE3%2B%84%9EA%8E