DRTVWR-418: Fix minor merge mistake.
There was a merge collision in multi.c's multi_runsingle() function at the point where we have a local patch concerning bug 1420. The upstream change was to change a Curl_done() call to multi_done(). Our patch called Curl_done() in each clause of an 'if' statement; we changed both to multi_done(). However, our patch also had a Curl_posttransfer() call; we overzealously changed that to multi_posttransfer() too. But there is no such function. Change back to Curl_posttransfer().