Move assert in dump_dir() in zdb

Move assert in dump_dir() in zdb

This one line patch moves an assert in the function dump_dir()
below an error check that ensures it ran correctly. This ensures
zdb dumps the error that actually caused the problem, as opposed
to one of its symptoms.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #8171