[gvfs/gnome-3-34] dnssd: guard avahi_client_free to prevent freeing NULL

[Ondrej Holy <oholy src gnome org>]

commit e41e2d0d26f6e9b6824dd98eef98be34d1cb834b
Author: Tobias Mueller <muelli cryptobitch de>
Date:   Wed Nov 27 20:06:42 2019 +0000

    dnssd: guard avahi_client_free to prevent freeing NULL
    
    This should not happen, but somehow the global_client ends up
    being NULL.
    In daemon/gvfsbackenddnssd.c the call to avahi_client_free in
    the finalize function is similarly guarded.
    
    
    (cherry picked from commit a36f8846731f07ec7ff38cfa3ff1a233ea929481)

 common/gvfsdnssdresolver.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)
---
diff --git a/common/gvfsdnssdresolver.c b/common/gvfsdnssdresolver.c
index 8782f5fc..0f0f5f60 100644
--- a/common/gvfsdnssdresolver.c
+++ b/common/gvfsdnssdresolver.c
@@ -172,7 +172,10 @@ free_global_avahi_client (void)
   g_list_foreach (resolvers, (GFunc) remove_client_from_resolver, NULL);
 
   /* Destroy client */
-  avahi_client_free (global_client);
+  if (global_client)
+    {
+      avahi_client_free (global_client);
+    }
   global_client = NULL;
   avahi_initialized = FALSE;
 }