hw/timer/arm_timer: Avoid array overrun for bad addresses

The integrator's timer read/write functions log an error for
bad addresses in guest accesses, but were falling through and
using an out of bounds array index rather than returning early.
Fix this.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Message-id: 1392647854-8067-4-git-send-email-peter.maydell@linaro.org
Cc: qemu-stable@nongnu.org
