Fix Itertools::k_smallest on unfused iterators

In the case of an unfused iterator yielding at most k-1 elements, then None, then other elements, the heap is not k-long and `for_each` is called on the other elements leading `debug_assert_eq` to rightfully panic.
Our recent variants of `k_smallest` do not have this bug!

Author: Philippe-Cholet

src/lib.rs

@@ -2965,15 +2965,17 @@ pub trait Itertools: Iterator {
 
         let mut heap = self.by_ref().take(k).collect::<BinaryHeap<_>>();
 
-        self.for_each(|i| {
-            debug_assert_eq!(heap.len(), k);
-            // Equivalent to heap.push(min(i, heap.pop())) but more efficient.
-            // This should be done with a single `.peek_mut().unwrap()` but
-            //  `PeekMut` sifts-down unconditionally on Rust 1.46.0 and prior.
-            if *heap.peek().unwrap() > i {
-                *heap.peek_mut().unwrap() = i;
-            }
-        });
+        if heap.len() == k {
+            self.for_each(|i| {
+                debug_assert_eq!(heap.len(), k);
+                // Equivalent to heap.push(min(i, heap.pop())) but more efficient.
+                // This should be done with a single `.peek_mut().unwrap()` but
+                //  `PeekMut` sifts-down unconditionally on Rust 1.46.0 and prior.
+                if *heap.peek().unwrap() > i {
+                    *heap.peek_mut().unwrap() = i;
+                }
+            });
+        }
 
         heap.into_sorted_vec().into_iter()
     }