Fix possible segfault in CopyDataRaws().
Fix possible segfault in CopyDataRaws().

Coverity pointed out that if pool_get_id() returns an error,
VALID_BACKEND will access out of array.  Per Coverity report 1111413
"Memory - illegal accesses".

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