treefmt . at the root of the project fails if the project root is a symlink

[jfly]

Describe the bug

If the root of your project is a symlink, and you try running treefmt . at the root of the repo, you'll get a segfault.

To Reproduce

Steps to reproduce the behavior:

1. Create a project ./prj with a working treefmt configuration.
2. Create a symlink ./prj-link: ln -s prj prj-link.
3. cd prj-link
4. treefmt .

   $ treefmt .
   panic: runtime error: invalid memory address or nil pointer dereference
   [signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x6c0548]
   
   goroutine 1 [running]:
   github.com/numtide/treefmt/v2/walk.(*CompositeReader).Read(0xc000214180, {0x9775a8?, 0xc0002042a0?}, {0xc00022c008?, 0x1?, 0x85bf20?})
       github.com/numtide/treefmt/v2/walk/walk.go:165 +0x88
   github.com/numtide/treefmt/v2/cmd/format.Run(0x4?, 0xc0000aeaa0, 0xc0001cbc48?, {0xc0000d5da0, 0x1, 0x3})
       github.com/numtide/treefmt/v2/cmd/format/format.go:157 +0x8e6
   github.com/numtide/treefmt/v2/cmd.runE(0xc0000ea480, 0xc0000aeaa0, 0xc0001bc008, {0xc0000d5da0, 0x1, 0x3})
       github.com/numtide/treefmt/v2/cmd/root.go:183 +0x6c6
   github.com/numtide/treefmt/v2/cmd.NewRoot.func1(0xc000180500?, {0xc0000d5da0?, 0x7?, 0x8c0546?})
       github.com/numtide/treefmt/v2/cmd/root.go:37 +0x32
   github.com/spf13/cobra.(*Command).execute(0xc0001bc008, {0xc000014110, 0x3, 0x3})
       github.com/spf13/[email protected]/command.go:1015 +0xaaa
   github.com/spf13/cobra.(*Command).ExecuteC(0xc0001bc008)
       github.com/spf13/[email protected]/command.go:1148 +0x46f
   github.com/spf13/cobra.(*Command).Execute(0xc000002380?)
       github.com/spf13/[email protected]/command.go:1071 +0x13
   main.main()
       github.com/numtide/treefmt/v2/main.go:12 +0x18

Expected behavior

We certainly shouldn't crash. I would expect this to format the whole repo despite the fact that the user have given us a path to a symlink, which is in opposition to the intent/thinking behind #528.

System information

N/A

Additional context

This was broken in #528. Root cause:

1. This line of code skips over the given path if it's a symlink: https://github.com/numtide/treefmt/blob/v2.3.1/walk/walk.go#L284
2. That leaves us with a nil entry in this readers array.
3. We pull the nil value out of the readers array here: https://github.com/numtide/treefmt/blob/v2.3.1/walk/walk.go#L160
4. We try to call c.current.Read where c.current == nil here: https://github.com/numtide/treefmt/blob/v2.3.1/walk/walk.go#L165. Boom.