[PGPRO-11557] Fix infinite loop with multiple ORDER BY in RUM scan

[LimarDeWills]

Fixes #66
Problem description:

When executing a query with ORDER BY col1, col2 using a RUM index, and the WHERE clause matches only a subset of the index, the query may enter an infinite loop in scanGetItemFast().

This occurs because entry->curItem is set via entry->curItem = collected.item;, but the corresponding entry->offset does not point to that item in entry->list. As a result, entryFindItem() may prematurely exit without correcting curItem, leading to repeated processing of the same item.
Fix:

Added an extra check in entryFindItem() that ensures:

curItem >= item

offset is within valid range

entry->curItem.iptr == entry->list[offset].iptr

If the last condition fails, the function continues scanning the list to realign curItem and offset.

This ensures scanGetItemFast() progresses correctly and prevents infinite looping.