Fix memory leak for parallel tests

[MattCatz]

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

• Issues fixed (if any):
  congestion_used is set each iteration of the loop. For tests running parallel streams, the previous malloc-ed string (from strdup) is leaked.

• Brief description of code changes (suitable for use as a commit message):
  If congestion_used is not NULL, free the existing string.

[bmah888]

Thanks for the PR! At first glimpse this looks like a good change to make (rather, two good changes to make).

[bmah888]

Looks good...merging after brief testing.