BUG/MEDIUM: peers: fix partial message decoding

BUG/MEDIUM: peers: fix partial message decoding

peer_recv_msg() may return because the message is incomplete without
checking if a shutdown is pending for the SC. The function relies on
co_getblk() to detect shutdowns. However, the message length decoding may be
interrupted if the multi-bytes integer is incomplete. In this case, the SC
is not check for shutdowns.

When this happens, this leads to an appctx spinning loop.

This patch should fix the issue #2373. It must be backported to 2.8.