Fix crash when GetAllChildren returns nil,nil

[zagge-cgeo]

When GetAllChildren return nil, nil the following panic happens

panic: runtime error: makeslice: cap out of range

goroutine 1 [running]:
iso9660.(*File).GetChildren(0x54ec60?)

/home/tzachmann/develop/test/go/iso9660/src/iso9660/image_reader.go:254 +0x51

Here is the code that is problematic. When GetAllChildren returns nil, nil make with a size of -2 will be called which results in the above panic.

func (f *File) GetChildren() ([]*File, error) {
     children, err := f.GetAllChildren()
     if err != nil {
         return nil, err
     }

     filteredChildren := make([]*File, 0, len(children)-2)

This patch returns an error in case there are no children or the ReadAt fails.

If in this cases a nil, nil is fine the above code could be changed to

 filteredChildren := make([]*File, 0, len(children))

to fix the problem.

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (08dd38c) 78.94% compared to head (470883c) 78.74%.

❗ Current head 470883c differs from pull request most recent head 51c2226. Consider uploading reports for the commit 51c2226 to get more accurate results

Files	Patch %	Lines
image_reader.go	0.00%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #48      +/-   ##
==========================================
- Coverage   78.94%   78.74%   -0.20%     
==========================================
  Files           6        6              
  Lines        1211     1214       +3     
==========================================
  Hits          956      956              
- Misses        180      182       +2     
- Partials       75       76       +1     

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[kdomanski]

Great catch! Thanks for the patch.
Do you think you could add a testcase or two? Otherwise I'll take care of it after the holidays.

[zagge-cgeo]

Do you think you could add a testcase or two?

Unfortunately I have no clue how to test this.