src: Fix division by zero in UDP bandwidth calculation in iperf_api.c

[AntonMoryakov]

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

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

  • In iperf_api.c, variables sender_time and receiver_time were used as denominators in bandwidth calculations without a zero check, which could lead to division by zero and undefined behavior.
  • To fix this, introduced sent_bandwidth and recv_bandwidth variables with a conditional check to prevent division by zero. If time is zero, bandwidth is now safely set to 0.0.

[bmah888]

Looks good to me. Thanks for the PR!