server: malloc/free fix

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/server.c b/server.c
index 8f93d2f..648758e 100644
--- a/server.c
+++ b/server.c
@@ -302,6 +302,7 @@
 			break;
 
 		free(cmd);
+		cmd = NULL;
 	}
 
 	if (cmd)