[efi] Check the status code from AllocatePool()

[efi] Check the status code from AllocatePool()

According to the latest UEFI specification (Version 2.8 Errata B)
p. 7.2:

  "Buffer: A pointer to a pointer to the allocated buffer if the call
   succeeds; undefined otherwise."

So implementations are obliged neither to return NULL, if the
allocation fails, nor to preserve the contents of the pointer.

Make the logic more reliable by checking the status code from
AllocatePool() instead of checking the returned pointer for NULL

Signed-off-by: Ignat Korchagin <ignat@cloudflare.com>
Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>