mgmt/mcumgr/lib: Fix cbor_internal_read_object error overwrite

The cbor_value_advance at the end of attribute parsing loop could
overwrite err with 0; this could cause loop to fail to break
on error and make cbor_internal_read_object return success, even
if parsing of input buffer failed.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>