Autocompletion makes the shell panic when there's no suggestion

[asdine]

genji> EXPLAIN SELECT a, b FROM foo WHERE a != panic: runtime error: index out of range [0] with length 0

goroutine 1 [running]:
github.com/genjidb/genji/cmd/genji/shell.(*Shell).completer(0xc00013a3c0, 0xc000030320, 0x45, 0x29, 0x994940, 0xc00013a401, 0xc0002ed580)
        /home/asdine/code/github.com/genjidb/genji/cmd/genji/shell/shell.go:380 +0x5e6
github.com/c-bata/go-prompt.(*CompletionManager).Update(...)
        /home/asdine/go/pkg/mod/github.com/c-bata/go-prompt@v0.2.3/completion.go:69
github.com/c-bata/go-prompt.(*Prompt).Run(0xc000142400)
        /home/asdine/go/pkg/mod/github.com/c-bata/go-prompt@v0.2.3/prompt.go:95 +0x6cd
github.com/genjidb/genji/cmd/genji/shell.Run(0xc000129620, 0xc000129620, 0xc000138840)
        /home/asdine/code/github.com/genjidb/genji/cmd/genji/shell/shell.go:114 +0x365
main.main.func1(0xc000162840, 0x0, 0xc00014a6e0)
        /home/asdine/code/github.com/genjidb/genji/cmd/genji/main.go:52 +0x165
github.com/urfave/cli.HandleAction(0x95e0a0, 0xa22000, 0xc000162840, 0xc000162840, 0x0)
        /home/asdine/go/pkg/mod/github.com/urfave/cli@v1.22.4/app.go:526 +0xfd
github.com/urfave/cli.(*App).Run(0xc000252000, 0xc0001181b0, 0x1, 0x1, 0x0, 0x0)
        /home/asdine/go/pkg/mod/github.com/urfave/cli@v1.22.4/app.go:288 +0x655
main.main()
        /home/asdine/code/github.com/genjidb/genji/cmd/genji/main.go:58 +0x1d4

This happens when the parser returns a parseError with no suggestion in it. A check must be added before using parser suggestions.