Integrate known fixes for PROPACK problems

[asl]

PROPACK code is known to contain some bugs that may lead to out-of-bounds memory reads and corresponding problems.

You may want to integrate the following two commits that fixes them (note that these are for double version, so, they need to be ported to other variants and implementations):

1. asl/svd@ac3f496
2. asl/svd@b6f2c09

[ilayn]

Hi @asl Not sure how we missed this one, but we did. Apologies for that. We are now in the process of replacing PROPACK with its C rewrite and I incorporated these fixes into the C code. The pull request is not open yet but the work is going closing to a finish in my local branch. You can see the C project structure here https://github.com/ilayn/scipy/tree/rewrite_propack/scipy/sparse/linalg/_propack I'll add some Readme and other license details soon.

Thank you for the heads up.