Fix memory allocation hazard (#1542).

[bmah888]

Reported by: @someusername123 on GitHub

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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): Closes DoS on sending invalid length in iperf_api.c+2684 #1542

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

[swlars]

Looks good!

[swlars]

Thanks @someusername123 for the bug report!