Prevent `Curl_pgrsTime` from modifying `t_starttransfer` when invoked
with `TIMER_STARTTRANSFER` more than once during a single request.
When a redirect occurs, this is considered a new request and
`t_starttransfer` can be updated to reflect the `t_starttransfer` time
of the redirect request.
Closes#1616
Bug: https://github.com/curl/curl/pull/1602#issuecomment-310267370
