Fix "treeRow is undefined" error dragging to bottom of collections list
Fix "treeRow is undefined" error dragging to bottom of collections list

If you drag past the collections list the row is -1.

Fixes #958

rmzelle/zotero,gracile-fr/zotero,retorquere/zotero,retorquere/zotero,rmzelle/zotero,retorquere/zotero,gracile-fr/zotero,rmzelle/zotero,gracile-fr/zotero