Ensure there's at least one selection in Editor::set_selections

This commit introduces an assertion that will cause Zed to panic as
soon as the invariant gets violated. This will be useful to investigate
issue #503.

Co-Authored-By: Nathan Sobo <nathan@zed.dev>

crates/editor/src/editor.rs

@@ -4992,6 +4992,11 @@ impl Editor {
         pending_selection: Option<PendingSelection>,
         cx: &mut ViewContext<Self>,
     ) {
+        assert!(
+            !selections.is_empty() || pending_selection.is_some(),
+            "must have at least one selection"
+        );
+
         let old_cursor_position = self.newest_anchor_selection().head();
 
         self.selections = selections;