Fix read_kind_from_backend() possible segfault when all backend down.
Fix read_kind_from_backend() possible segfault when all backend down.

Per Coverity report 1111414 "Negative array index read".

treasure-data/prestogres,treasure-data/prestogres,treasure-data/prestogres,treasure-data/prestogres,treasure-data/prestogres,treasure-data/prestogres,treasure-data/prestogres