Prevent server termination with JSON and remove redundant code with error

[davidBar-On]

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

• Issues fixed (if any): Server side JSON problem #1631

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

Fix to two issues when JSON is used, one of them caused server termination:

1. iperf_errexit() locked the mutex, but called iperf_json_finish() that also lock/unlock the mutex, so when the client terminated before the end of the test, the mutex unlock failed and the server terminated.
2. Removed a redundant code, which was probably forgotten after copying it from some lines above. The redundant code caused redundant client error when it terminated.

[bmah888]

Thanks for the PR! Initial tests with your patch are looking good so far, but I want to go back through the code (along with your commentary at the top of this PR) to understand how we (I) managed to mess this up without noticing. :-p

[bmah888]

Thanks again for the PR! OK, I understand the original problems now. One change I think we need is to interchange the order of the mutex lock/unlock operations that you just moved/added. I think fixing these bugs uncovered some other problems (locking in the --json-stream codepaths and duplicated JSON output in the non --json-stream output codepath). I'm going to try to figure out fixes for these, not sure if I want to try to add to this PR or create a new PR for them.

[davidBar-On]

I switched between the lock and unlock, but if other related changes are needed, then I agree it is better to have a new PR (I don't have a problem with this).

[bmah888]

Thanks, OK I think this PR is good to go now! I'm hoping to have a fix for the other two problems shortly.