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

lapack-netlib/SRC/sbdsqr.f

@@ -410,12 +410,12 @@
    40    CONTINUE
    50    CONTINUE
          SMINOA = SMINOA / SQRT( REAL( N ) )
-         THRESH = MAX( TOL*SMINOA, MAXITR*N*N*UNFL )
+         THRESH = MAX( TOL*SMINOA, MAXITR*(N*(N*UNFL)) )
       ELSE
 *
 *        Absolute accuracy desired
 *
-         THRESH = MAX( ABS( TOL )*SMAX, MAXITR*N*N*UNFL )
+         THRESH = MAX( ABS( TOL )*SMAX, MAXITR*(N*(N*UNFL)) )
       END IF
 *
 *     Prepare for main iteration loop for the singular values