properly check malloc return value

Reference: /n/sources/patch/applied/sokoban-malloc
Date: Sat May 10 23:43:36 CES 2008
Signed-off-by: fshahriar@gmail.com

--- /sys/src/games/sokoban/route.c	Sat May 10 23:42:51 2008
+++ /sys/src/games/sokoban/route.c	Sat May 10 23:42:50 2008
@@ -85,7 +85,7 @@
 	Walk *w;
 
 	w = malloc(sizeof(Walk));
-	if (w->route == nil)
+	if (w == nil)
 		sysfatal("cannot allocate walk");
 	memset(w, 0, sizeof(Walk));
 	return w;