Fix bug 608: the sign computation in LDLT was broken
(transplanted from 7df582d9b7d1986c1db1e4dadfe20581f38b5adb)

--HG--
branch : 3.1
extra : transplant_source : %7D%F5%82%D9%B7%D1%98l%1D%B1%E4%DA%DF%E2%05%81%F3%8BZ%DB