(Windows) Fix error on joining paths due to undefined home dir

Use more robust way of getting home directory. The code is taken from AWS
library to have parity in logic.

lib/backend.js

@@ -3,6 +3,7 @@ const _ = require('lodash')
 const {promisify} = require('es6-promisify')
 const path = require('path')
 const fs = require('fs')
+const os = require('os')
 const errorhandler = require('errorhandler')
 const { extractKey, extractKeysForItems, parseKey } = require('./util')
 const bodyParser = require('body-parser')
@@ -88,15 +89,33 @@ const createAwsConfig = (AWS) => {
   return dynamoConfig
 }
 
+function getHomeDir() {
+  const env = process.env
+  const home = env.HOME || env.USERPROFILE
+      || (env.HOMEPATH ? ((env.HOMEDRIVE || 'C:/') + env.HOMEPATH) : null)
+
+  if (home) {
+    return home
+  }
+
+  if (typeof os.homedir === 'function') {
+    return os.homedir()
+  }
+
+  return null
+}
+
 exports.createServer = (dynamodb, docClient) => {
   const app = express()
   app.set('json spaces', 2)
   app.set('view engine', 'ejs')
   app.set('views', path.resolve(__dirname, '..', 'views'))
 
   if (!dynamodb || !docClient) {
-    if (fs.existsSync(path.join(process.env.HOME, '.aws', 'credentials')) &&
-        fs.existsSync(path.join(process.env.HOME, '.aws', 'config'))) {
+    const homeDir = getHomeDir()
+
+    if (homeDir && fs.existsSync(path.join(homeDir, '.aws', 'credentials')) &&
+        fs.existsSync(path.join(homeDir, '.aws', 'config'))) {
       process.env.AWS_SDK_LOAD_CONFIG = 1
     }