fix: skip extract if linkpath is stripped entirely

Fix tar.Unpack() to skip extraction of hardlinks and symlinks when a
'strip' option is provided, if the entry linkpath would be completely
stripped.

Previously, the linkpath would not be stripped if it had fewer path parts
than the strip option.

This matches the behavior of modern versions of bsdtar.  Gnutar has the
same extraction semantics, but emits a warning when the resulting
linkpath is completely stripped.

lib/unpack.js

@@ -209,6 +209,8 @@ class Unpack extends Parser {
         const linkparts = normPath(entry.linkpath).split('/')
         if (linkparts.length >= this.strip)
           entry.linkpath = linkparts.slice(this.strip).join('/')
+        else
+          return false
       }
     }
 

test/unpack.js

@@ -191,16 +191,8 @@ t.test('links!', t => {
     t.end()
   }
   const checkForStrip3 = t => {
-    t.ok(fs.lstatSync(dir + '/3').isDirectory())
-    let err = null
-    try {
-      fs.lstatSync(dir + '/3/hardlink-3')
-    } catch(e) {
-      err = e
-    }
-    // can't be extracted because we've passed it in the tar
-    // (specially crafted tar for this not to work)
-    t.equal(err.code, 'ENOENT')
+    // strips the linkpath entirely, so the link doesn't get extracted.
+    t.throws(() => fs.lstatSync(dir + '/3'), { code: 'ENOENT' })
     t.end()
   }