JSON_Read() warning messages enhancements

[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): none

• Brief description of code changes (suitable for use as a commit message):
  Enhancement to the JSON_Read() warning() messages done while evaluating issue iperf3: error - unable to receive parameters from client: Bad file descriptor #1808. I believe that they can help in other situations.

[bmah888]

Thanks for the PR! Just one quick comment so far: it'd probably be better to use snprintf() rather than sprintf() in this patch to limit the number of bytes that get written into msg_buf. So maybe something like snprintf(msg_buf, sizeof(msg_buf), ...

The chance of overflowing msg_buf with the current code is probably zero, but the use of snprintf(3) is generally discouraged and I think some compilers will warn about its usage.

[davidBar-On]

... it'd probably be better to use snprintf() rather than sprintf() ...

Changed sprintf() to snprintf()