allow empty user names in SSH protocol

diff --git a/openssh b/openssh
index 5d21250..ac0ba1f 160000 (submodule)
--- a/openssh
+++ b/openssh
@@ -1 +1 @@
-Subproject commit 5d212506d4ccc7f0e93dbfc756f380583addb20e
+Subproject commit ac0ba1f390586dd0300f0a036ce30952b1dd5def