fix(sevenzip): add nil guard for subreader to prevent panic

When opening a sevenzip archive, the init() method calls sr :=SevenZipReader()
but never checked if sr was nil before using it. This could cause a nil
pointer dereference when processing malformed or empty archives. Add an
explicit nil check returning errFormat early if the subreader is nil.

Also fixes a minor import grouping whitespace issue.

internal/sevenzip/reader.go

@@ -19,6 +19,7 @@ import (
 
 	"bookhoard/internal/sevenzip/internal/pool"
 	"bookhoard/internal/sevenzip/internal/util"
+
 	"github.com/bodgit/plumbing"
 	"github.com/spf13/afero"
 )
@@ -415,6 +416,10 @@ func (z *Reader) init(r io.ReaderAt, size int64) (err error) {
 		return err
 	}
 
+	if sr == nil {
+		return errFormat
+	}
+
 	// Work out where we are in the file (32, avoiding magic numbers)
 	if z.start, err = sr.Seek(0, io.SeekCurrent); err != nil {
 		return fmt.Errorf("sevenzip: error seeking current position: %w", err)