rgw/rgw_metadata.h: init cur_shard in LogListCtx with 0

CID 1054868 (#1 of 1): Uninitialized scalar field (UNINIT_CTOR)
  uninit_member: Non-static class member "cur_shard" is not
  initialized in this constructor nor in any functions that it calls.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>