Fix various bugs found by scan-build and refresh scan-build config in CI

See the commit messages for details. This helps #1767 (closed) but doesn’t fix it, as there are still a number of scan-build reports during compilation. All those reports (at least with my local testing) are false positives, and I am putting together a second MR to squash those. I’m keeping it separate from this one, though, so that this one can have a bit more scrutiny (it includes functional changes) and to make backporting the right fixes a little easier.

Signed-off-by: Philip Withnall pwithnall@gnome.org

Helps: #1767 (closed)