outdated: Ensure errors reading the package tree do not crash

Fixes: #13115
Credit: @iarna
Reviewed-By: @othiym23
PR-URL: #13121

lib/outdated.js

@@ -76,6 +76,7 @@ function outdated (args, silent, cb) {
   if (npm.config.get('depth') === Infinity) npm.config.set('depth', 0)
 
   readPackageTree(dir, andRecalculateMetadata(function (er, tree) {
+    if (!tree) return cb(er)
     mutateIntoLogicalTree(tree)
     outdated_(args, '', tree, {}, 0, function (er, list) {
       list = uniq(list || []).sort(function (aa, bb) {

test/tap/outdated-bad-read-tree.js

@@ -0,0 +1,21 @@
+'use strict'
+var test = require('tap').test
+var requireInject = require('require-inject')
+var npm = require('../../lib/npm.js')
+
+test('setup', function (t) {
+  npm.load({progress: false}, t.end)
+})
+
+test('outdated', function (t) {
+  var rptError = new Error('read-package-tree')
+  var outdated = requireInject('../../lib/outdated.js', {
+    'read-package-tree': function (dir, cb) {
+      cb(rptError)
+    }
+  })
+  outdated([], function (err) {
+    t.is(err, rptError)
+    t.end()
+  })
+})