workspace: Sanitize pinned tab count before usage (#21417)

Fixes all sorts of panics around usage of incorrect pinned tab count
that has been fixed in app itself, yet persists in user db.

Closes #ISSUE

Release Notes:

- N/A

crates/workspace/src/pane.rs

@@ -1890,7 +1890,7 @@ impl Pane {
     fn unpin_tab_at(&mut self, ix: usize, cx: &mut ViewContext<'_, Self>) {
         maybe!({
             let pane = cx.view().clone();
-            self.pinned_tab_count = self.pinned_tab_count.checked_sub(1).unwrap();
+            self.pinned_tab_count = self.pinned_tab_count.checked_sub(1)?;
             let destination_index = self.pinned_tab_count;
 
             let id = self.item_for_index(ix)?.item_id();