BUG/MINOR: peers: Possible appctx pointer dereference.

This bug may occur when enabling peers traces. It is possible that
peer->appctx is NULL when entering peer_session_release().

(cherry picked from commit 4b1a05fcf81b4343d29aeb86394d44f29d2116a4)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
diff --git a/src/peers.c b/src/peers.c
index a530a33..a38fc3c 100644
--- a/src/peers.c
+++ b/src/peers.c
@@ -425,7 +425,7 @@
 			const struct peer *peer = a2;
 			struct peers *peers = NULL;
 
-			if (peer) {
+			if (peer && peer->appctx) {
 				struct stream_interface *si;
 
 				si = peer->appctx->owner;