Fix a crash that can be triggered by libiperf3 clients that don't use
iperf_parse_arguments().  Basically we need to initialize the
output stream in the iperf_test structure regardless of whether
iperf_parse_arguments() gets called; some programs (in particular
the programs in the examples/ directory and bwctl) don't do this
(and indeed should not need to).

This problem was introduced in the solution for Issue #119; the fix
needs to be merged to any codeline where fixes for Issue #119 go.