panic: runtime error: index out of range [-1]

[knweiss]

I found a testcase that causes a runtime panic in gci:

$ cat config/config_fuzz.go 
package config

import (
//      "os"
)
$ gci config
panic: runtime error: index out of range [-1]

goroutine 1 [running]:
github.com/daixiang0/gci/pkg/gci.(*pkg).fmt(0xc00011c0a0, 0x1, 0x1, 0x0)
        .../go/src/github.com/daixiang0/gci/pkg/gci/gci.go:130 +0xb0a
github.com/daixiang0/gci/pkg/gci.processFile(0xc000126020, 0x15, 0x51fbc0, 0xc000128008, 0xc00011c040, 0x0, 0x0)
        .../go/src/github.com/daixiang0/gci/pkg/gci/gci.go:297 +0x33a
github.com/daixiang0/gci/pkg/gci.visitFile.func1(0xc000126020, 0x15, 0x520b60, 0xc000110b60, 0x0, 0x0, 0x4aec7d, 0xc000110b60)
        .../go/src/github.com/daixiang0/gci/pkg/gci/gci.go:249 +0xb3
path/filepath.walk(0xc000126020, 0x15, 0x520b60, 0xc000110b60, 0xc000104290, 0x0, 0x0)
        .../go/go/src/path/filepath/path.go:360 +0x423
path/filepath.walk(0x7ffed58acf99, 0x6, 0x520b60, 0xc000110a90, 0xc000104290, 0x0, 0xc00010cec0)
        .../go/go/src/path/filepath/path.go:384 +0x2fe
path/filepath.Walk(0x7ffed58acf99, 0x6, 0xc000104290, 0x0, 0x0)
        .../go/go/src/path/filepath/path.go:406 +0x105
github.com/daixiang0/gci/pkg/gci.WalkDir(0x7ffed58acf99, 0x6, 0xc00011c040, 0xc0001109c0, 0x0)
        .../go/src/github.com/daixiang0/gci/pkg/gci/gci.go:256 +0x4d
main.main()
        .../go/src/github.com/daixiang0/gci/main.go:56 +0x1c5

AFAICS len(ret) is 0 in gci/pkg/gci/gci.go:

   130          if ret[len(ret)-1] == linebreak {
   131                  ret = ret[:len(ret)-1]
   132          }