Fix panic in file finder path elision (#25658)

Release Notes:

- N/A

crates/file_finder/src/file_finder.rs

@@ -909,7 +909,9 @@ impl FileFinderDelegate {
                 (normal, small)
             };
             let budget = full_path_budget(&file_name, normal_em, small_em, max_width);
-            if full_path.len() > budget {
+            // If the computed budget is zero, we certainly won't be able to achieve it,
+            // so no point trying to elide the path.
+            if budget > 0 && full_path.len() > budget {
                 let components = PathComponentSlice::new(&full_path);
                 if let Some(elided_range) =
                     components.elision_range(budget - 1, &full_path_positions)