Keep any text killed in the command prompt with C-w and yank it with

C-y, only use the top buffer if no text has previously been killed. This
and previous change promped by discussion with kn@.

server-client.c

@@ -302,6 +302,7 @@ server_client_lost(struct client *c)
 		free(msg);
 	}
 
+	free(c->prompt_saved);
 	free(c->prompt_string);
 	free(c->prompt_buffer);