Fix assert bug in sparseQR
(grafted from bd3786d250645508611c2605c1733eb29158053c)

--HG--
branch : 3.2
extra : source : bd3786d250645508611c2605c1733eb29158053c
Reduce explicit zeros when applying SparseQR's matrix Q
(grafted from 744b1d67f0d11e288614e9db56147e666a5eb5aa)

--HG--
branch : 3.2
extra : source : 744b1d67f0d11e288614e9db56147e666a5eb5aa