[Privet Filesystem] Fix null dereference

Fix null dereference issue in privet filesystem if listing a directory errors
out. Instead, signal an error.

BUG=

Review URL: https://codereview.chromium.org/180733006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255430 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/local_discovery/storage/privet_filesystem_operations.cc

@@ -149,15 +149,15 @@ void PrivetFileSystemListOperation::OnStorageListResult(
   DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
   list_operation_.reset();
 
-  attribute_cache_->AddFileInfoFromJSON(full_path_, value);
-
-  fileapi::AsyncFileUtil::EntryList entry_list;
-
   if (!value) {
     SignalError();
     return;
   }
 
+  attribute_cache_->AddFileInfoFromJSON(full_path_, value);
+
+  fileapi::AsyncFileUtil::EntryList entry_list;
+
   const base::ListValue* entries;
   if (!value->GetList(kPrivetListEntries, &entries)) {
     SignalError();