MINOR: logs: Add support of checks as session origin to format lf strings

When a log-format string is built from an health-check, the session origin
is the health-check itself and not a connection. In addition, there is no
stream. It means for now some formats are not supported: %s, %sc, %b, %bi,
%bp, %si and %sp.

Thanks to this patch, the session origin is converted to a check. So it is
possible to retrieve the backend and the backend connection. Note this
session have no listener, thus %ft format must be guarded.

This patch is light and standalone, thus it may be backported as far as 2.2
if required. However, because the error is human, it is probably better to
wait a bit to be sure everything is properly protected.
diff --git a/src/log.c b/src/log.c
index 7930c52..79166bf 100644
--- a/src/log.c
+++ b/src/log.c
@@ -2188,10 +2188,10 @@
 		 * A request error is reported as it's the only element we have
 		 * here and which justifies emitting such a log.
 		 */
-		be = fe;
+		be = ((obj_type(sess->origin) == OBJ_TYPE_CHECK) ? __objt_check(sess->origin)->proxy : fe);
 		txn = NULL;
 		fe_conn = objt_conn(sess->origin);
-		be_conn = NULL;
+		be_conn = ((obj_type(sess->origin) == OBJ_TYPE_CHECK) ? cs_conn(__objt_check(sess->origin)->cs) : NULL);
 		status = 0;
 		s_flags = SF_ERR_PRXCOND | SF_FINST_R;
 		uniq_id = _HA_ATOMIC_FETCH_ADD(&global.req_count, 1);
@@ -2518,7 +2518,7 @@
 				if (iret == 0)
 					goto out;
 				tmplog += iret;
-				if (sess->listener->bind_conf->xprt == xprt_get(XPRT_SSL))
+				if (sess->listener && sess->listener->bind_conf->xprt == xprt_get(XPRT_SSL))
 					LOGCHAR('~');
 				if (tmp->options & LOG_OPT_QUOTE)
 					LOGCHAR('"');
@@ -2561,13 +2561,18 @@
 				break;
 
 			case LOG_FMT_SERVER: // %s
-				switch (obj_type(s ? s->target : NULL)) {
+				switch (obj_type(s ? s->target : sess->origin)) {
 				case OBJ_TYPE_SERVER:
 					src = __objt_server(s->target)->id;
 					break;
 				case OBJ_TYPE_APPLET:
 					src = __objt_applet(s->target)->name;
 					break;
+				case OBJ_TYPE_CHECK:
+					src = (__objt_check(sess->origin)->server
+					       ? __objt_check(sess->origin)->server->id
+					       : "<NOSRV>");
+					break;
 				default:
 					src = "<NOSRV>";
 					break;
@@ -2768,9 +2773,21 @@
 				break;
 
 			case LOG_FMT_SRVCONN:  // %sc
-				ret = ultoa_o(objt_server(s ? s->target : NULL) ?
-				                 objt_server(s->target)->cur_sess :
-				                 0, tmplog, dst + maxsize - tmplog);
+				switch (obj_type(s ? s->target : sess->origin)) {
+				case OBJ_TYPE_SERVER:
+					ret = ultoa_o(__objt_server(s->target)->cur_sess,
+						      tmplog, dst + maxsize - tmplog);
+					break;
+				case OBJ_TYPE_CHECK:
+					ret = ultoa_o(__objt_check(sess->origin)->server
+						      ? __objt_check(sess->origin)->server->cur_sess
+						      : 0, tmplog, dst + maxsize - tmplog);
+					break;
+				default:
+					ret = ultoa_o(0, tmplog, dst + maxsize - tmplog);
+					break;
+				}
+
 				if (ret == NULL)
 					goto out;
 				tmplog = ret;