i2c: Add a length check to the SMBus write handling

i2c: Add a length check to the SMBus write handling

Avoid an overflow.

Signed-off-by: Corey Minyard <cminyard@mvista.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Cc: QEMU Stable <qemu-stable@nongnu.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
(cherry picked from commit 629457a13080052c575779e1fd9f5eb5ee6b8ad9)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>