Bound the results-exchange JSON_read to reject an oversized length

[pau-hedgehog]

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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): Fixes get_results() passes 0 as max_size to JSON_read(), so a misread control-channel byte can trigger a multi-GB allocation instead of a clean error #2072

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

get_results() passed 0 as max_size to JSON_read(), disabling its bounds check on the results-exchange read. This adds a new MAX_RESULTS_JSON_STRING bound (64 MiB, generous enough for --get-server-output on a long, many-stream test) and uses it in that call, matching the treatment the params-read call already gets from MAX_PARAMS_JSON_STRING a few hundred lines earlier in the same file.

Tested locally: built and ran make check (all 5 unit tests pass), then ran a real client/server pair with the patched binary covering bidir multi-stream transfer and --get-server-output, confirming get_results() still parses legitimate results correctly under the new bound.