mute-console: refuse multiple requests per connection

Otherwise, that triggers memory leak and multiple free on the same
object.

src/mute-console/mute-console.c

@@ -236,6 +236,10 @@ static int vl_method_mute(
         assert(link);
         assert(FLAGS_SET(flags, SD_VARLINK_METHOD_MORE));
 
+        /* Refuse multiple requests per connection. */
+        if (sd_varlink_get_userdata(link))
+                return -EBUSY;
+
         _cleanup_free_ Context *nc = new(Context, 1);
         if (!nc)
                 return -ENOMEM;