nul-terminate cookie read from control connection in iperf_accept

[dxbjavid]

• 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):

on the server, iperf_accept() reads exactly COOKIE_SIZE bytes from the client's control connection into test->cookie (a char[COOKIE_SIZE]) but never forces a terminating NUL, whereas a well behaved client only sends 36 characters plus a trailing NUL. A peer that sends 37 non-NUL bytes leaves the buffer unterminated, and it is then handled as a C string in iperf_on_connect() (copied into the JSON cookie field and printed via report_cookie), so strlen reads past the array into the following struct members. This terminates the cookie right after it is read so the later string handling stays in bounds.

[bmah888]

Thanks for the bug report and PR! We'll take a look.