Fix backward compatibility issue of client incorrectly print 0 UDP packets loss

[davidBar-On]

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

• Issues fixed (if any): iperf3 client summary reports 0% UDP loss despite non-zero loss in server output #1984

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

Fix backward compatibility issue of client showing UDP 0 packets loss, although packets were lost. The issue is with older iperf3 servers that did not send omitted packets info.

The main issue was that for some reason, the omitted error count variable was not auto initialized to zero. However, when I added a printf() of its value, it was initialized to 0. Maybe a compiler issue, as the later initialization of the variable is done under if statement with no else.

For robustness, also ignored the omitted values if omitting time is not specified. That allows showing the real packets loss when server does not support sending the omitting info.

[bmah888]

Thanks for the PR! We'll review and comment.

[bmah888]

I see this problem when doing UDP tests from a relatively new iperf3 to an older iperf3 (such as iperf3-darwin, which seems to ble based on 8.9.1). Need to set a sending rate fast enough trigger some packet loss. The server counts missing packets, but the client reports that the server had no loss.

This PR fixes the issue.