`-write` removes trailing newline from modified file

[sim642]

Running reanalyze https://github.com/rescript-association/reanalyze/commit/389dd682223201b42c33e5c444e5c79e0805adad on Goblint https://github.com/goblint/analyzer/commit/a544002114324a207df72f9e79eb656c3ebe7b5b revealed the following.

When -write modifies a file to add dead code attributes, it removes the trailing newline from the file. Since all other code style is nicely preserved, it shouldn't be removing that.