Eigen_3.3.7.patch

diff --git a/Eigen/src/SVD/BDCSVD.h b/Eigen/src/SVD/BDCSVD.h
index 1134d66e7..bb9944b41 100644
--- a/Eigen/src/SVD/BDCSVD.h
+++ b/Eigen/src/SVD/BDCSVD.h
@@ -921,7 +921,17 @@ void BDCSVD<MatrixType>::perturbCol0
         Index i = perm(l);
         if(i!=k)
         {
-          Index j = i<k ? i : perm(l-1);
+          //Sometimes we get i >= k and l == 0, leading to perm(l-1) being out of bounds
+          //Here we make sure that perm isn't accessed out of bounds.
+          //However, when this happens, the resulting U,S and V^T matrices will usually contain
+          //NAN's, but at least we then get a chance to do something about it, instead of segfault.
+
+          //Index j = i<k ? i : perm(l-1);
+          Index j;
+          if (i<k) j = i;
+          else if (l >  0 and l < m) j = perm(l-1);
+          else continue;
+
           prod *= ((singVals(j)+dk) / ((diag(i)+dk))) * ((mus(j)+(shifts(j)-dk)) / ((diag(i)-dk)));
 #ifdef EIGEN_BDCSVD_DEBUG_VERBOSE
           if(i!=k && std::abs(((singVals(j)+dk)*(mus(j)+(shifts(j)-dk)))/((diag(i)+dk)*(diag(i)-dk)) - 1) > 0.9 )