Avoid needless hash table churn.

When reallocating, check that the pointer changed before updating the
hash table.  Otherwise, we're just removing then re-adding the same
value.  This frequently happens when the allocation is shrinking, or
only growing by a few bytes.

lib/xmalloc.c

@@ -338,7 +338,7 @@ xrealloc_impl(void *ptr, size_t new_size, const char *file, int line,
     xmalloc_fail_after--;
 
   new_ptr = realloc(ptr, new_size);
-  if (new_ptr != NULL)
+  if (new_ptr != NULL && new_ptr != ptr)
     {
       hash_table_del(xmalloc_table, ptr);
       hash_table_add(xmalloc_table, new_ptr, (int)new_size, file, line, func);