Fix crash in Windows PipeReader

[paperdave]

What does this PR do?

Fixes #10660, a crash impacting Next.js dev server while reading files.

To make better panics, I'm introducing two new functions:

• unexpectedTag, for use in a switch else => |t| unexpectedTag
• releaseNonNull, named after the method of the same name in WebKit's Ref. This is a release-mode safety checked version of .? and should be used if one is not confident

This fixes the test locally for me, but I don't think it will fix CI as CI was running into a different test failure.

[paperdave]

Does not fix the crash

[github-actions]

❌ @paperdave, your commit has failing tests :(

💪 2 failing tests Darwin AARCH64

• test/cli/install/bun-install.test.ts 1 failing
• test/js/node/worker_threads/worker_threads.test.ts SIGTRAP

💻 1 failing tests Darwin x64 baseline

• test/js/node/http2/node-http2.test.js 1 failing

💻 1 failing tests Darwin x64

• test/cli/install/registry/bun-install-registry.test.ts 2 failing

🐧💪 3 failing tests Linux AARCH64

• test/cli/install/registry/bun-install-registry.test.ts 1 failing
• test/js/bun/util/filesink.test.ts 1 failing
• test/js/node/http2/node-http2.test.js 1 failing

🐧🖥 3 failing tests Linux x64 baseline

• test/js/node/child_process/child_process-node.test.js 1 failing
• test/js/node/http2/node-http2.test.js 1 failing
• test/js/web/websocket/websocket.test.js 1 failing

🐧🖥 1 failing tests Linux x64

• test/js/node/http2/node-http2.test.js 1 failing

🪟💻 8 failing tests Windows x64 baseline

• test/cli/install/bunx.test.ts 1 failing
• test/cli/install/registry/bun-install-registry.test.ts 7 failing
• test/integration/next-pages/test/dev-server-ssr-100.test.ts 1 failing
• test/integration/next-pages/test/dev-server.test.ts 1 failing
• test/integration/next-pages/test/next-build.test.ts 1 failing
• test/js/bun/shell/bunshell.test.ts 1 failing
• test/js/node/dns/node-dns.test.js 2 failing
• test/js/web/workers/worker.test.ts 1 failing

🪟💻 10 failing tests Windows x64

• test/cli/install/bunx.test.ts 1 failing
• test/cli/install/migration/migrate.test.ts 1 failing
• test/integration/next-pages/test/dev-server-ssr-100.test.ts 1 failing
• test/integration/next-pages/test/dev-server.test.ts 1 failing
• test/integration/next-pages/test/next-build.test.ts 1 failing
• test/js/bun/shell/bunshell.test.ts 1 failing
• test/js/node/child_process/child_process.test.ts 1 failing
• test/js/node/dns/node-dns.test.js 2 failing
• test/js/node/http2/node-http2.test.js 1 failing
• test/js/node/watch/fs.watchFile.test.ts 1 failing

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