make Heap::as_slice not panic on empty heap

Author: TheMagzuz

src/dary/heap.rs

@@ -207,7 +207,11 @@ where
     /// assert!(slice.contains(&("z", 99)));
     /// ```
     pub(crate) fn as_slice(&self) -> &[(N, K)] {
-        &self.tree[offset::<D>()..]
+        if self.tree.is_empty() {
+            &self.tree
+        } else {
+            &self.tree[offset::<D>()..]
+        }
     }
 }
 

tests/priority_queue_tests/as_slice.rs

@@ -20,6 +20,7 @@ pub fn test_as_slice<const D: usize>(mut pq: DaryHeap<usize, f64, D>) {
     pq.clear();
     assert!(pq.is_empty());
     let mut vec = vec![];
+    assert!(pq.as_slice().is_empty());
 
     let mut rng = rand::rng();
     for node in 0..N {