Be more truthful about network availability

If flatpak-info exists, but we don't have the shared key, then
the network is not available. Pointed out by Cosimo.

gio/gportalsupport.c

@@ -61,6 +61,8 @@ glib_network_available_in_sandbox (void)
           available = g_strv_contains ((const char * const *)shared, "network");
           g_strfreev (shared);
         }
+      else
+        available = FALSE;
     }
 
   g_key_file_free (keyfile);