server.c: test **truncated_send_len** for partial

Testing against truncated_send_malloc won't work if we've already
buffered and completed a partial buffer, as truncated_send_malloc
isn't free'd until the socket is destroyed.
defer PONG send until no partial send buffer

Reported-by: Andrew Canaday
Signed-off-by: Andy Green <andy.green@linaro.org>
Merge branch 'master' into nytd_build