linux-imx

lguest: fix BUG_ON() in invalid guest page table.

If we discover the entry is invalid, we kill the guest, but we must
avoid calling gpte_addr() on the invalid pmd, otherwise:

	kernel BUG at drivers/lguest/page_tables.c:157!

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>