BUG/MEDIUM: peers: Properly handle shutdown when trying to get a line

BUG/MEDIUM: peers: Properly handle shutdown when trying to get a line

When a shutdown was reported to a peer applet, the event was not properly
handled if it failed to receive data. The function responsible to get data
was exiting too early if the applet buffer was empty, without testing the
sedesc status. Because of this issue, it was possible to have frozen peer
applets. For instance, it happend on client timeout. With too many frozen
applets, it was possible to reach the maxconn.

This patch should fix the issue #3234. It must be backported to 3.3.