[vino] Return empty string instead of NULL to prevent criticals

[Ondrej Holy <oholy src gnome org>]

commit 15eb3c5e51b799aa22fff547dd88921d9cc4b5f2
Author: Ondrej Holy <oholy redhat com>
Date:   Fri Aug 24 14:30:14 2018 +0200

    Return empty string instead of NULL to prevent criticals
    
    The code expects that avahi_client_get_host_name_fqdn never return NULL,
    but it can happen in some cases. Return empty string instead of NULL to
    prevent the following criticals:
    
    GLib-CRITICAL **: 14:29:52.305: g_variant_new_string: assertion 'string != NULL' failed
    
    https://bugzilla.gnome.org/show_bug.cgi?id=796349

 server/vino-mdns.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)
---
diff --git a/server/vino-mdns.c b/server/vino-mdns.c
index 4f7e693..6a16b64 100644
--- a/server/vino-mdns.c
+++ b/server/vino-mdns.c
@@ -356,7 +356,12 @@ vino_mdns_shutdown (void)
 const char *
 vino_mdns_get_hostname (void)
 {
-  return mdns_client ? avahi_client_get_host_name_fqdn (mdns_client) : "";
+  const char *hostname = NULL;
+
+  if (mdns_client)
+    hostname = avahi_client_get_host_name_fqdn (mdns_client);
+
+  return hostname ? hostname : "";
 }
 
 #else /* !defined (VINO_HAVE_AVAHI) */