Bug 1020766 - fix [Flatfish][Gallery]Gallery app shows last-deleted photo in  the preview window

https://bugzilla.mozilla.org/show_bug.cgi?id=1020766
Below comment explains this change well. I will recommend adding it above line 666, 
// On large devices we need to display the new current file after deletion.
// But if we just deleted the last file then we don't do this.