iperf_api: memset entire malloc

[RABijl]

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

• Issues fixed (if any):

• Brief description of code changes (suitable for use as a commit message):
  in the function iperf_new_test
  the bitrate_limit_intervals_traffic_bytes array
  was only memset for the size of the sizeof
  return type, instead of the entire array.

while we're on this topic, is there are reason why calloc is not used but instead of malloc + memset?

[bmah888]

Thanks for the PR! Yeah that's a pretty silly bug isn't it.