[xhci] Fix length of allocated slot array

The xHCI slot ID is one-based, not zero-based.  Fix the length of the
xhci->slot[] array to account for this, and add assertions to check
that the hardware returns a valid slot ID in response to the Enable
Slot command.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/usb/xhci.c

 		rc = id;
 		goto err_enable_slot;
 	}
+	assert ( ( id > 0 ) && ( id <= xhci->slots ) );
 	assert ( xhci->slot[id] == NULL );
 
 	/* Allocate and initialise structure */
 	int rc;
 
 	/* Allocate device slot array */
-	xhci->slot = zalloc ( xhci->slots * sizeof ( xhci->slot[0] ) );
+	xhci->slot = zalloc ( ( xhci->slots + 1 ) * sizeof ( xhci->slot[0] ) );
 	if ( ! xhci->slot ) {
 		rc = -ENOMEM;
 		goto err_slot_alloc;
 
 	/* Sanity checks */
 	assert ( xhci->slot != NULL );
-	for ( i = 0 ; i < xhci->slots ; i++ )
+	for ( i = 0 ; i <= xhci->slots ; i++ )
 		assert ( xhci->slot[i] == NULL );
 
 	xhci_stop ( xhci );