Curl_pp_readresp: zero terminate line

The comment in the code mentions the zero terminating after having
copied data, but it mistakingly zero terminated the source data and not
the destination! This caused the test 864 problem discussed on the list:

http://curl.haxx.se/mail/lib-2013-12/0113.html
Signed-off-by: Daniel Stenberg <daniel@haxx.se>