Bashate warned on init-swift.sh because of E043 -- arithmetic
compound has inconsistent return semantics: (( next_port++ ))
New Zuul reported that on every change as a warning.
This is fixed here.

This change makes Bashate always produce errors so that we do not
introduce such warnings again.

Change-Id: I40166b377ec2580e17901375b636183bca492d3a