target/s390x: avoid integer overflow in next_page PC check

target/s390x: avoid integer overflow in next_page PC check

If the PC is in the last page of the address space, next_page_start
overflows to 0. Fix it.

Reviewed-by: Richard Henderson <[email protected]>
Reviewed-by: Thomas Huth <[email protected]>
Reviewed-by: David Hildenbrand <[email protected]>
Acked-by: Cornelia Huck <[email protected]>
Cc: Cornelia Huck <[email protected]>
Cc: Alexander Graf <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: [email protected]
Signed-off-by: Emilio G. Cota <[email protected]>
Signed-off-by: Richard Henderson <[email protected]>