Fix crash on null vhost-user socket - VPP-573

Fix for VPP-573.

Change-Id: If7d9690901efebf62fdf28219097153d98c79c0c
Signed-off-by: Wojciech Dec <wdec@cisco.com>
diff --git a/src/vnet/devices/virtio/vhost-user.c b/src/vnet/devices/virtio/vhost-user.c
index bde8106..f9bbae4 100644
--- a/src/vnet/devices/virtio/vhost-user.c
+++ b/src/vnet/devices/virtio/vhost-user.c
@@ -2578,6 +2578,11 @@
   int rv = 0;
   int server_sock_fd = -1;
 
+  if (sock_filename == NULL || !(strlen (sock_filename) > 0))
+    {
+      return VNET_API_ERROR_INVALID_ARGUMENT;
+    }
+
   if (is_server)
     {
       if ((rv =