Fix integer overflow in LAPACK DBDSQR, SBDSQR (#1135)

Fix integer overflow in LAPACK DBDSQR, SBDSQR (#1135)

* Fix integer overflow in DBDSQR

As noted in lapack issue 135, an integer overflow in the calculation of the iteration limit could lead to an immediate return without any iterations having been performed if the input matrix is sufficiently big.

* Fix integer overflow in SBDSQR

As noted in lapack issue 135, an integer overflow in the calculation of the iteration limit could lead to an immediate return without any iterations having been performed if the input matrix is sufficiently big.

* Fix integer overflow in threshold calculation

Related to lapack issue 135, the threshold calculation can overflow as well as the multiplication is evaluated from left to right.
Without explicit parentheses, the calculation would overflow for N >= 18919

* Fix integer overflow in threshold calculation

Related to lapack issue 135, the threshold calculation can overflow as well as the multiplication is evaluated from left to right.
Without explicit parentheses, the calculation would overflow for N >= 18919