App crash when reusing the same DataSource twice

[SergioEstevao]

Expected behavior

• The picker should open correctly every time.

Actual behavior

• After using one time the picker the second time it's presented we get a crash

Steps to reproduce the behavior

Change this code in the DemoController line 240 before you test:

self.pickerDataSource = [[WPPHAssetDataSource alloc] init]; to
self.pickerDataSource = [WPPHAssetDataSource sharedInstance];

• Open the picker in full screen mode
• Close it
• Open it again

On the current demo opening the picker didn't cause the bug because we where a creating new data source each time.

Refs: wordpress-mobile/WordPress-iOS#7988

Tested on iPhone7, iOS 11, WPMediaPicker 0.23