Additional safety check when checking for persisted item pane state

I don't think there's any way for this to happen short of manual
preference editing, and the pane width would have to be exactly 250px,
and it's pretty harmless anyway, but might as well be safe.

chrome/content/zotero/zoteroPane.js

@@ -4093,6 +4093,7 @@ var ZoteroPane = new function()
 				if (el.id == 'zotero-items-splitter') continue;
 				// And don't restore to min-width if splitter was collapsed
 				if (el.id == 'zotero-item-pane' && attr == 'width' && elValues[attr] == 250
+						&& 'zotero-items-splitter' in serializedValues
 						&& serializedValues['zotero-items-splitter'].state == 'collapsed') {
 					continue;
 				}