fix error when pipeline starts with parallel section and adds a subsequent stage (regression)
fix error when pipeline starts with parallel section and adds a
subsequent stage (regression)

tucano/bpipe,sb43/bpipe,tucano/bpipe,vivovip/bpipe,sb43/bpipe,sb43/bpipe,vivovip/bpipe,tucano/bpipe,vivovip/bpipe