common: Fix commit f99830b.

* common/userids.c (classify_user_id): Avoid underflow.  Use spacep to
also trim tabs.

This is actually not fully consistent because the now used
trim_trailing_spaces uses the locale dependent isspace and not spacep.
Given that the use of isspace is anyway problematic we should check
whether we can chnage trim_trailing_spaces.

• Signed-off-by: Werner Koch <wk@gnupg.org>