utils: check npm.config existence in error-handler.js (#108)

Fixes: https://npm.community/t/npx-envinfo-preset-jest-fails-on-windows-with-a-stack-trace/2713
PR-URL: #108
Credit: @BeniCheni
Reviewed-By: @zkat

Author: BeniCheni

lib/utils/error-handler.js

@@ -202,7 +202,7 @@ function errorHandler (er) {
   msg.summary.concat(msg.detail).forEach(function (errline) {
     log.error.apply(log, errline)
   })
-  if (npm.config.get('json')) {
+  if (npm.config && npm.config.get('json')) {
     var error = {
       error: {
         code: er.code,