server: Silence compiler warning in fd_queue_async().

server/fd.c

diff --git a/server/fd.c b/server/fd.c
index 656042f..4ad4da5 100644
--- a/server/fd.c
+++ b/server/fd.c
@@ -1735,6 +1735,7 @@
         queue = fd->wait_q;
         break;
     default:
+        queue = NULL;
         assert(0);
     }