Fix Windows Portable App mode

Fixes: #971

This is a perfect example of putting too many statements into a
try-catch block. My bad. I was trying to keep the code simple, but it
bit us here.

This happens because we were using IS_PRODUCTION, but the order of the
consts at the top are:

const IS_PORTABLE = isPortable()
const IS_PRODUCTION = isProduction()

So we're inside of isPortable() and referring to IS_PRODUCTION before
it's defined. This should have thrown an exception, since const does
not allow use-before-define, but we're transforming to ES5 with Babel.

Also, standard could have caught this, but we can't enable the
use-before-define rule until this bug is fixed:
https://github.com/feross/standard/issues/636

Basically, a perfect storm.

src/config.js

@@ -12,8 +12,8 @@ const IS_TEST = isTest()
 const PORTABLE_PATH = IS_TEST
   ? path.join(process.platform === 'win32' ? 'C:\\Windows\\Temp' : '/tmp', 'WebTorrentTest')
   : path.join(path.dirname(process.execPath), 'Portable Settings')
-const IS_PORTABLE = isPortable()
 const IS_PRODUCTION = isProduction()
+const IS_PORTABLE = isPortable()
 
 const UI_HEADER_HEIGHT = 38
 const UI_TORRENT_HEIGHT = 100
@@ -141,11 +141,18 @@ function isPortable () {
   if (IS_TEST) {
     return true
   }
-  try {
-    return process.platform === 'win32' && IS_PRODUCTION && !!fs.statSync(PORTABLE_PATH)
-  } catch (err) {
+
+  if (process.platform !== 'win32' || !IS_PRODUCTION) {
+    // Fast path: Non-Windows platforms should not check for path on disk
     return false
   }
+
+  let portablePathExists = false
+  try {
+    portablePathExists = !!fs.statSync(PORTABLE_PATH)
+  } catch (err) {}
+
+  return portablePathExists
 }
 
 function isProduction () {