Fixes a clip bug in pg_stat_get_backend_activity()

Hi Hackers,

This is an eyeball caught bug:
```
clipped_activity = pgstat_clip_activity(activity);
ret = cstring_to_text(activity); <== bug here: should use clipped_activity
pfree(clipped_activity);

PG_RETURN_TEXT_P(ret);
```
clipped result is stored in clipped_activity, but it's not used.

This should be an edge case bug. It would only be triggered by using
multi-byte characters in SQL and the SQL statement length
exceeds track_activity_query_size, and a multi-byte character just across
the boundary and gets truncated in the middle.

I tried to reproduce the bug, but failed. Anyway, this is an obvious fix,
no logic change.

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