ensure zeroing the cache_object_t structure when creating a new one

fixes and edge case where uninitialized memory would have been accessed
(basically accessing obj->dlen when fetch_from_peer() fails because of connectivity problems)

==21156== Thread 58:
==21156== Conditional jump or move depends on uninitialised value(s)
==21156==    at 0x4199E4: arc_move (arc.c:172)
==21156==    by 0x41A3E5: arc_lookup (arc.c:377)
==21156==    by 0x417981: shardcache_get (shardcache.c:703)
==21156==    by 0x412135: shardcached_handle_get_request (shardcached.c:315)
==21156==    by 0x412614: shardcached_request_handler (shardcached.c:435)
==21156==    by 0x40E800: handle_request (mongoose.c:4456)
==21156==    by 0x410814: process_new_connection (mongoose.c:5086)
==21156==    by 0x410DA6: worker_thread (mongoose.c:5186)
==21156==    by 0x5039B4F: start_thread (pthread_create.c:304)