cl: guard zero-sized interface deref

[cpunion]

Summary

• guard zero-sized pointer dereferences when they are boxed into interfaces
• add test/go coverage for issue19246 behavior
• remove fixedbugs/issue19246.go xfails for darwin/arm64 and linux/amd64

Tests

• go test ./test/goroot -run '^TestGoRootRunCases$/fixedbugs/issue19246.go$' -count=1 -args -goroot "/opt/homebrew/Cellar/go@1.24/1.24.11/libexec" -dirs fixedbugs -case '^fixedbugs/issue19246.go$' -xfail /tmp/llgo-empty-xfail.yaml -run-timeout 30s
• go test ./test/go -count=1
• go test ./cl ./ssa -count=1 -p=1
• (runtime submodule) go test ./internal/runtime -count=1

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.

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