Fix potential upgrade error from c963637

If a related item was defined using both an old userID and the current
one, the schema upgrade step could fail.

chrome/content/zotero/xpcom/schema.js

@@ -2424,7 +2424,7 @@ Zotero.Schema = new function(){
 						if (matches) {
 							// Wrong libraryID
 							if (matches[1] != userID) {
-								yield Zotero.DB.queryAsync(`UPDATE itemRelations SET object='http://zotero.org/users/${userID}/items/${matches[2]}' WHERE itemID=? AND predicateID=?`, [row.itemID, predicateID]);
+								yield Zotero.DB.queryAsync(`UPDATE OR REPLACE itemRelations SET object='http://zotero.org/users/${userID}/items/${matches[2]}' WHERE itemID=? AND predicateID=?`, [row.itemID, predicateID]);
 							}
 						}
 					}