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: NRusty Russell <rusty@rustcorp.com.au>

drivers/lguest/page_tables.c

@@ -669,8 +669,10 @@ unsigned long guest_pa(struct lg_cpu *cpu, unsigned long vaddr)
 
 #ifdef CONFIG_X86_PAE
 	gpmd = lgread(cpu, gpmd_addr(gpgd, vaddr), pmd_t);
-	if (!(pmd_flags(gpmd) & _PAGE_PRESENT))
+	if (!(pmd_flags(gpmd) & _PAGE_PRESENT)) {
 		kill_guest(cpu, "Bad address %#lx", vaddr);
+		return -1UL;
+	}
 	gpte = lgread(cpu, gpte_addr(cpu, gpmd, vaddr), pte_t);
 #else
 	gpte = lgread(cpu, gpte_addr(cpu, gpgd, vaddr), pte_t);