handle another out-of-memory in fork(). the policy here is not consistent,

since other calls (e.g. exec()) panic on out of memory.

kernel/proc.c

@@ -161,6 +161,8 @@ proc_pagetable(struct proc *p)
 
   // An empty page table.
   pagetable = uvmcreate();
+  if(pagetable == 0)
+    return 0;
 
   // map the trampoline code (for system call return)
   // at the highest user virtual address.