cadence_uart: bounds check write offset

cadence_uart_init() initializes an I/O memory region of size 0x1000
bytes.  However in uart_write(), the 'offset' parameter (offset within
region) is divided by 4 and then used to index the array 'r' of size
CADENCE_UART_R_MAX which is much smaller: (0x48/4).  If 'offset>>=2'
exceeds CADENCE_UART_R_MAX, this will cause an out-of-bounds memory
write where the offset and the value are controlled by guest.

This will corrupt QEMU memory, in most situations this causes the vm to
crash.

Fix by checking the offset against the array size.

Cc: qemu-stable@nongnu.org
Reported-by: 李强 <liqiang6-s@360.cn>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Message-id: 20160418100735.GA517@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>