stdlib: Avoid another self-comparison in qsort

In the insertion phase, we could run off the start of the array if the
comparison function never runs zero.  In that case, it never finds the
initial element that terminates the iteration.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

stdlib/qsort.c

@@ -217,7 +217,7 @@ insertion_sort_qsort_partitions (void *const pbase, size_t total_elems,
   while ((run_ptr += size) <= end_ptr)
     {
       tmp_ptr = run_ptr - size;
-      while (cmp (run_ptr, tmp_ptr, arg) < 0)
+      while (run_ptr != tmp_ptr && cmp (run_ptr, tmp_ptr, arg) < 0)
         tmp_ptr -= size;
 
       tmp_ptr += size;