notmuch: Plug a leaked notmuch DB handle in get_count()

nm_nonctx_get_count() was leaking its notmuch database handle if NOTMUCH_API_3
was in use. Looks like it missed the same treatment that the other call sites
got to do this.
Merge branch 'get_count_db_leak' of https://github.com/tsto/mutt-kz

* 'get_count_db_leak' of https://github.com/tsto/mutt-kz:
  notmuch: Plug a leaked notmuch DB handle in get_count()