BUG/MEDIUM: httpclient: don't consume data before it was analyzed

BUG/MEDIUM: httpclient: don't consume data before it was analyzed

In httpclient_applet_io_handler(), on the response path, we don't check
if the data are in the output part of the channel, and could consume
them before they were analyzed.

To fix this issue, this patch checks for the stline and the headers if
the msg_state is >= HTTP_MSG_DATA which means the stline and headers
were analyzed. For the data part, it checks if each htx blocks is in the
output before copying it.

Must be backported in 2.5.