Fix memory leak in gist_page_items() of pageinspect

Hi Hackers,

While reading the code of pageinspect, I just found a memory leak
in gist_page_items():

```
values[4] = CStringGetTextDatum(buf.data);
nulls[4] = false;
```

where CStringGetTextDatum() has made a copy of buf.data and assigned to
value[4], however buf.data is never free-ed. This leak is inside a
per-tuple loop, thus it should be fixed.

In the meantime, the other small issue was also found in the same function.
An index is opened by index_open() but closed by index_close() and
relation_close() in different places. I also fixed the problem by changing
relation_close() to index_close().

Best regards,
==
Chao Li (Evan)
---------------------
HighGo Software Co., Ltd.
https://www.highgo.com/