Do not allow setting zero parallel streams

[davidBar-On]

• Version of iperf3 (or development branch, such as master or
  3.1-STABLE) to which this pull request applies:
  master 3.21+

• Issues fixed (if any): none

• Brief description of code changes (suitable for use as a commit message):

Dot not allow --parallel to be zero (noticed this issue as the latest verification change caused a conflict in an old PR I submitted). Changed the error message accordingly.

(Also includes a cosmetic changes: indentation fix and changing }else to } else.)

[bmah888]

Thanks for the PR! This used to be a part of PR #2045, right?

[davidBar-On]

This used to be a part of PR #2045, right?

Yes. I separated the PRs as it seems that merging of PR #2045 is not straightforward as I thought it is, at least not its testing.

[bmah888]

Thanks, this code change looks good.

In the future, I'd like to request not putting whitespace cleanups in the same PR as bugfixes or other functional changes. Or at least not put them in the same commits. This makes it harder to tell exactly what the significant changes were in each commit. Thanks!