stdlib: Fix heapsort for cases with exactly two elements

When malloc fails to allocate a buffer and falls back to heapsort, the
current heapsort implementation does not perform sorting when there are
exactly two elements. Heapsort is now skipped only when there is
exactly one element.

Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

stdlib/qsort.c

@@ -162,7 +162,7 @@ get_swap_type (void *const pbase, size_t size)
 static void
 heapsort_r (void *base, size_t n, size_t size, __compar_d_fn_t cmp, void *arg)
 {
-  if (n <= 1)
+  if (n == 0)
     return;
 
   enum swap_type_t swap_type = get_swap_type (base, size);