Fix two small bugs in new gistget.c logic.

1. Complain, rather than silently doing nothing, if an "invalid" tuple
is found on a leaf page.  Per off-list discussion with Heikki.

2. Fix oversight in code that removes a GISTSearchItem from the search
queue: we have to reset lastHeap if this was the last heap item in the
parent GISTSearchTreeItem.  Otherwise subsequent additions will do the
wrong thing.  This was probably masked in early testing because in typical
cases the parent item would now be completely empty and would be deleted on
next call.  You'd need a queued non-leaf page at exactly the same distance
as a heap tuple to expose the bug.