x86_64: pmap: add null-pointer to check to delete_pdir

arch/x86_64/pmap.c

@@ -92,6 +92,7 @@ static void delete_ptab(phys_addr_t pt)
 
 	pt &= ~VM_PAGE_MASK;
 	if (!pt) {
+		/* physical address of 0x0, nothing to delete */
 		return;
 	}
 	
@@ -107,6 +108,10 @@ static void delete_pdir(phys_addr_t pd)
 	}
 
 	pd &= ~0x1FFFFFULL;
+	if (!pd) {
+		/* physical address of 0x0, nothing to delete */
+		return;
+	}
 		
 	pdir_t *pdir = vm_phys_to_virt(pd);
 	for (int i = 0; i < 512; i++) {