malloc: harden removal from unsorted list

* malloc/malloc.c (_int_malloc): Added check before removing from
unsorted list.

malloc/malloc.c

@@ -3775,6 +3775,8 @@ _int_malloc (mstate av, size_t bytes)
             }
 
           /* remove from unsorted list */
+          if (__glibc_unlikely (bck->fd != victim))
+            malloc_printerr ("malloc(): corrupted unsorted chunks 3");
           unsorted_chunks (av)->bk = bck;
           bck->fd = unsorted_chunks (av);