Re: checking snapshot argument for index_beginscan

Hi,

I was looking at commit 941aa6a6268a6a66f6895401aad6b5329111d412 .

I think it would be better to move the assertion into `index_beginscan_internal` because it is called by index_beginscan and index_beginscan_bitmap

In the future, when a new variant of `index_beginscan_XX` is added, the assertion would be effective for the new variant.

Please let me know what you think.

Cheers