fix(linear-summarization): Fix segfault bug in get_linear_summary

[tonywu1999]

This dry run build is failing due to a segfault that is only caught in the CI/CD workflow. It seems that the issue is the use of single & instead of double && to perform boolean IF statement checks. See this post on the differences and why it matters.

Just remember that if you encounter a problem in life, even a segfault, you can do anything. You can even fly.

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[tonywu1999]

Debugging process:

1. Pushed a dummy commit to verify that it was a problem on the devel branch as well.
2. Thought it had to do with the ubuntu migration based on this article and this post. Downgraded ubuntu to 22.04, but that didn't work.
3. Turned off any vignettes / tests related to linear summarization, and bugs disappeared, indicating it was isolated to the linear summarization code.
4. Sprinkled debug C++ print statements Rcout << "Input vector: " << x << std::endl;. Discovered line of code that was causing the issue.
5. Identified this stackoverflow post on differences between & and &&

I still have no idea why this error started popping up today but went undetected in the past. Plus it's still passing in our local machines.