Crash when attempting to output top-level value as Zeek TSV

[philrz]

Repro is where this issue first surfaced in Zed commit 524f500, which is associated with the changes in #3300. It's still with us at current tip of main, commit 81a9273. We became aware of this issue because the nightly perf-compare automation attempts a count() with output in Zeek TSV format, so the recent changes in #4420 made that script crash where it hadn't before.

$ zed -version
Version: v0.33.0-5-g524f5009

$ echo '"hello"' | zq -z -
"hello"

$ echo '"hello"' | zq -f zeek -
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0x11dc4ff]

goroutine 1 [running]:
github.com/brimdata/zed.(*Value).Columns(...)
	/Users/phil/work/zed/recordval.go:137
github.com/brimdata/zed/expr.(*Flattener).Flatten(0xc000370ef0, 0xc0003f4000)
	/Users/phil/work/zed/expr/flattener.go:73 +0x7f
github.com/brimdata/zed/zio/zeekio.(*Writer).Write(0xc000342a50, 0xc00004a0a0?)
	/Users/phil/work/zed/zio/zeekio/writer.go:44 +0x3d
github.com/brimdata/zed/zbuf.WriteBatch({0x2a96a318, 0xc000342a50}, {0x18de710?, 0xc00031a7b0?})
	/Users/phil/work/zed/zbuf/batch.go:37 +0x77
github.com/brimdata/zed/driver.(*CLI).Write(0xc00019f9e8?, 0xc00019f910?, {0x18de710?, 0xc00031a7b0?})
	/Users/phil/work/zed/driver/driver.go:207 +0x99
github.com/brimdata/zed/driver.run(0xc000373d10, {0x18df340, 0xc000373cb0}, 0xc00033f500, 0x0)
	/Users/phil/work/zed/driver/driver.go:151 +0x4be
github.com/brimdata/zed/driver.RunWithFileSystem({0x18df6f8?, 0xc000037b40?}, {0x18df340, 0xc000373cb0}, {0x18d71c0, 0xc000372ed0}, 0x1?, {0xc000370ec0, 0x1, 0x1}, ...)
	/Users/phil/work/zed/driver/driver.go:58 +0xe5
github.com/brimdata/zed/cmd/zed/query.(*Command).Run(0xc000156c60, {0xc0000372f0, 0x1, 0x1})
	/Users/phil/work/zed/cmd/zed/query/command.go:164 +0x74c
github.com/brimdata/zed/pkg/charm.path.run({0xc0002983d0?, 0x2, 0x2}, {0xc0000372f0?, 0x1, 0x0?})
	/Users/phil/work/zed/pkg/charm/path.go:11 +0x8b
github.com/brimdata/zed/pkg/charm.(*Spec).ExecRoot(0x165ba60?, {0xc0000372c0, 0x4, 0x4})
	/Users/phil/work/zed/pkg/charm/charm.go:63 +0x4a
main.main()
	/Users/phil/work/zed/cmd/zq/main.go:14 +0x1ea

[philrz]

I just came across this issue during a bug scrub and only then realized that I recently bumped into it again and opened a new issue #5107. Since that issue has two separate panic examples I'll close this one as a duplicate.