Fix UBSan with -i0: avoid division by zero

[jackyyf]

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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):

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

Fix UBSan with `-i0`: avoid division by zero

The interval is explicity allowed to be zero, but we are trying to
divide by the interval without checking if it's zero, which will cause
an UBSan failure.

This commit fixed the issue by verify the interval is not zero before
division.

• Actual UBSan report:

iperf3/src/iperf_api.c:1962:6: runtime error: inf is outside the range of representable values of type 'int'
    #0 0x555b386f6125 in iperf_parse_arguments iperf3/src/iperf_api.c:1962:6
    #1 0x555b386ef30a in main iperf3/src/main.c:117:9
<snip>

SUMMARY: UndefinedBehaviorSanitizer: float-cast-overflow iperf3/src/iperf_api.c:1962:6

[jackyyf]

Sorry, forgot to include the command line used:

iperf3 -clocalhost -i0

and iperf3 will fail when compiled with -fsanitize=float-cast-overflow