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.

src/trace.c

@@ -144,7 +144,7 @@ void __trace(enum trace_level level, uint64_t mask, struct trace_source *src,
 	}
 	if (check) {
 		srv = check->server;
-		be = srv->proxy;
+		be = (srv ? srv->proxy : NULL);
 	}
 
 	if (!srv && conn)