Ruby crash when `Fiber.scheduler` is active

[larskanis]

Our scheduler tests sometimes crash with a segfault in vm_search_cc. There is no appearance of pg_ext in the stack trace, so that it looks like a ruby bug rather than a bug in ruby-pg.

Examples are:

• https://github.com/ged/ruby-pg/actions/runs/3158834078/jobs/5141321924#step:12:105
• https://github.com/larskanis/ruby-pg/runs/4902305840?check_suite_focus=true#step:12:996
• https://github.com/larskanis/ruby-pg/runs/4820935863?check_suite_focus=true#step:11:995
• https://github.com/ged/ruby-pg/runs/6963745982?check_suite_focus=true#step:12:440

And sometimes ruby crashes within the fiber:

• https://github.com/larskanis/ruby-pg/runs/5152227530?check_suite_focus=true#step:12:335
• https://github.com/ged/ruby-pg/runs/5314898478?check_suite_focus=true#step:12:1037
• https://github.com/larskanis/ruby-pg/runs/5324756469?check_suite_focus=true#step:12:794

On Windows there's no stack trace, but it seems to happen there as well:

• https://github.com/larskanis/ruby-pg/runs/5152240485?check_suite_focus=true#step:12:788