Security: Check Github's Code scanning warnings

[hoffie]

Describe the bug
https://github.com/jamulussoftware/jamulus/security/code-scanning is non-empty

To Reproduce

Expected behavior

Clear out false positives, fix any genuine issues.

Screenshots

Additional context
I think most of them might be non-issues. Some are found in the bundled libopus.

cc @pljones as you asked whether this is tracked somewhere. Now it is. :)
cc @softins As one of your recent PRs is related to a specific instance of those warnings.

[softins]

Most of these are in array indices, where we are multiplying ints, but the default type for an array index is size_type, which is long. With the size of data that Jamulus deals with, a 32-bit int is more than plenty, and casting the first item in the expression to static_cast<long> in every affected location would just make the code verbose and less readable.

I would favour finding a way to switch off this particular CodeQL warning for the Jamulus repo.

[hoffie]

Looks like we can "Dismiss" these alerts at the above URL. I've just done that for everything in libs/ (opus and oboe). You can choose a reason. I chose WONTFIX (as we don't maintain these ourselves). Unless those alerts re-appear at some point, I'd say that this is the best way to solve them?

I don't feel confident about judging about the others myself, but I guess @softins does? :)