clear np->mem after freeing

proc.c

@@ -107,6 +107,7 @@ copyproc(struct proc* p)
   np->kstack = kalloc(KSTACKSIZE);
   if(np->kstack == 0){
     kfree(np->mem, np->sz);
+    np->mem = 0;
     np->state = UNUSED;
     return 0;
   }