BUG/MINOR: trace: Test server existence for health-checks to get proxy

Email alerts are based on health-checks but with no server. Thus, in
__trace() function, responsible to write a trace message, we must be
prepared to have no server and thus no proxy.

This patch must be backported as far as 2.4.

(cherry picked from commit e3b257479652bf148944571e8aca7beafd5afd65)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit cd26a7ff58cdb3f1d97a6a8b89a37d1c4160452b)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 9769a6a8da67ad5c50f4d6b99548c7492728c495)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
diff --git a/src/trace.c b/src/trace.c
index 4c5270c..ede3d0e 100644
--- a/src/trace.c
+++ b/src/trace.c
@@ -136,7 +136,7 @@
 	}
 	if (check) {
 		srv = check->server;
-		be = srv->proxy;
+		be = (srv ? srv->proxy : NULL);
 	}
 
 	if (!srv && conn)