BUG/MEDIUM: deinit: check fdtab before fdtab[fd].owner

When running a pure config check (haproxy -c) we go through the deinit
phase without having allocated fdtab, so we can't blindly dereference
it. The issue was added by recent commit ae7bc4a23 ("MEDIUM: deinit:
close all receivers/listeners before scanning proxies"), no backport is
needed.
diff --git a/src/haproxy.c b/src/haproxy.c
index 3bdcba7..4ef79bc 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -2452,7 +2452,7 @@
 	protocol_unbind_all();
 
 	for (cur_fd = 0; cur_fd < global.maxsock; cur_fd++) {
-		if (!fdtab[cur_fd].owner)
+		if (!fdtab || !fdtab[cur_fd].owner)
 			continue;
 
 		if (fdtab[cur_fd].iocb == listener_accept) {