Fix a segfaul in ni_testbus_client_claim_host_by_capability when not using a timeout

Signed-off-by: Olaf Kirch <okir@suse.de>
ni_process_run: when the client does not capture stderr separately, dup stdout to stderr

This used to work before, but got broken at some point.

Signed-off-by: Olaf Kirch <okir@suse.de>