post-processor/atlas: fix index out of range panic when artifacts are present

post-processor/atlas/post-processor.go

@@ -178,7 +178,7 @@ func (p *PostProcessor) PostProcess(ui packer.Ui, artifact packer.Artifact) (pac
 
 			// Modify the archive options to only include the files
 			// that are in our file list.
-			include := make([]string, 0, len(fs))
+			include := make([]string, len(fs))
 			for i, f := range fs {
 				include[i] = strings.Replace(f, path, "", 1)
 			}