DB backends broken if used on the client

[michaelkuhn]

It seems that the GError passed to, for example, j_db_schema_delete is passed all the way through to j_backend_db_schema_delete. If the original GError is freed in the meantime (which happens for the DB benchmarks due to _benchmark_db_prepare_scheme), the following assertion can be triggered:

j_backend_db_schema_delete: assertion 'error == NULL || *error == NULL' failed

This can be reproduced using ./scripts/benchmark.sh -p /db with a client-side mysql backend. (It also happens with the sqlite backend if it is first modified so it can be loaded on the client.)