MariaRocks: port my_hash_const_element

"Add new function my_hash_const_element(), the const equivalent of
my_hash_element()" - comes from facebook/mysql-5.6,
7c869d34b9fa2262b941efd6363a260b7c37948f

include/hash.h

@@ -74,6 +74,7 @@ my_bool my_hash_init2(HASH *hash, uint growth_size, CHARSET_INFO *charset,
 void my_hash_free(HASH *tree);
 void my_hash_reset(HASH *hash);
 uchar *my_hash_element(HASH *hash, ulong idx);
+const uchar *my_hash_const_element(const HASH *hash, ulong idx);
 uchar *my_hash_search(const HASH *info, const uchar *key, size_t length);
 uchar *my_hash_search_using_hash_value(const HASH *info,
                                        my_hash_value_type hash_value,

mysys/hash.c

@@ -721,6 +721,14 @@ uchar *my_hash_element(HASH *hash, ulong idx)
 }
 
 
+const uchar *my_hash_const_element(const HASH *hash, ulong idx)
+{
+  if (idx < hash->records)
+    return dynamic_element(&hash->array,idx,const HASH_LINK*)->data;
+  return 0;
+}
+
+
 /*
   Replace old row with new row.  This should only be used when key
   isn't changed