Panic when walkpgdir returns NULL in pagefault

Somewhat cleaner than hitting a page fault.

Panic when walkpgdir returns NULL in pagefault
de379bc2 · David Benjamin · 0a9b36be · de379bc2
--- a/kernel/vm.cc
+++ b/kernel/vm.cc
@@ -479,6 +479,8 @@ vmap::pagefault(uptr va, u32 err)
    return -1;

  atomic<pme_t> *pte = walkpgdir(pml4, va, 1);
+  if (pte == nullptr)
+    panic("pagefault: couldn't allocate page table");

 retry:
  pme_t ptev = pte->load();