[PATCH] Check snapshot argument of index_beginscan and family

Hi hackers,

A colleague of mine (cc'ed) reported that he was able to pass a NULL
snapshot to index_beginscan() and it even worked to a certain degree.

I took my toy extension [1] and replaced the argument with NULL as an
experiment:

```
eax=# CREATE EXTENSION experiment;
CREATE EXTENSION
eax=# SELECT phonebook_lookup_index('Alice');
phonebook_lookup_index
------------------------
-1
(1 row)

eax=# SELECT phonebook_insert('Bob', 456);
phonebook_insert
------------------
1
(1 row)

eax=# SELECT phonebook_lookup_index('Alice');
phonebook_lookup_index
------------------------
-1
(1 row)

eax=# SELECT phonebook_insert('Alice', 123);
phonebook_insert
------------------
2
(1 row)

eax=# SELECT phonebook_lookup_index('Alice');
server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.
```

So evidently it really works as long as the index doesn't find any
matching rows.

This could be really confusing for the extension authors so here is a
patch that adds corresponding Asserts().

[1]: https://github.com/afiskon/postgresql-extensions/tree/main/005-table-access

--
Best regards,
Aleksander Alekseev