typo bug in function bb_cache_users

[_ck_]

There is a typo bug in function bb_cache_users found in bb-includes/functions.php

which has existed since the first public bbPress release.

unset($ids[i]);

should be

unset($ids[$i]);

note the (lack of) $ on the i

This causes the cache to always miss on users since the search value never gets unset.

(Typo may exist in other similar functions, have not checked yet.)

(In [1384]) bb_cahce_users() typo - Fixes #843 - props _ck_