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.

kernel/vm.c

@@ -195,13 +195,14 @@ uvmunmap(pagetable_t pagetable, uint64 va, uint64 npages, int do_free)
 }
 
 // create an empty user page table.
+// returns 0 if out of memory.
 pagetable_t
 uvmcreate()
 {
   pagetable_t pagetable;
   pagetable = (pagetable_t) kalloc();
   if(pagetable == 0)
-    panic("uvmcreate: out of memory");
+    return 0;
   memset(pagetable, 0, PGSIZE);
   return pagetable;
 }