gpg: Fix uninitialized access to search descindex with gpg keyboxes.

* kbx/keybox-search.c (keybox_search): Add arg  R_DESCINDEX.  Chnage
both callers.
* g10/keydb.c (keydb_search): Always set DESCINDEX.

This only affects the new keybox for OpenPGP keys in 2.1. The bug
exhibited itself by running GPA's backup command on Windows.