Fix warning VS2017 15.7 update.

This previously unreported warning comes from casting size_t to ulong
in sql_hset.h in Hash_Set::at().

Change my_hash_element to accept size_t index parameter.

include/hash.h

@@ -73,7 +73,7 @@ my_bool my_hash_init2(HASH *hash, uint growth_size, CHARSET_INFO *charset,
                       uint flags);
 void my_hash_free(HASH *tree);
 void my_hash_reset(HASH *hash);
-uchar *my_hash_element(HASH *hash, ulong idx);
+uchar *my_hash_element(HASH *hash, size_t 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

@@ -748,7 +748,7 @@ my_bool my_hash_update(HASH *hash, uchar *record, uchar *old_key,
 }
 
 
-uchar *my_hash_element(HASH *hash, ulong idx)
+uchar *my_hash_element(HASH *hash, size_t idx)
 {
   if (idx < hash->records)
     return dynamic_element(&hash->array,idx,HASH_LINK*)->data;