kbx: Fix an endless loop under Windows due to an incomplete fix.

* kbx/keybox-search.c (keybox_search):  We need to seek to the last
position in all cases not just when doing a NEXT.

This is because search from the beginning needs a keybox_search_reset.
We can only make an exception for KEYDB_SEARCH_MODE_FIRST..

• Fixes-commit: 6f72aa821407e47ad3963e72e139f2ca2c69d9dd
• GnuPG-bug-id: T4505
• Signed-off-by: Werner Koch <wk@gnupg.org>