ipmi:smbus: Add a check around a memcpy

ipmi:smbus: Add a check around a memcpy

In one case:

  memcpy(sid->inmsg + sid->inlen, buf, len);

if len == 0 then sid->inmsg + sig->inlen can point to one past the inmsg
array if the array is full.  We have to allow len == 0 due to some
vagueness in the spec, but we don't have to call memcpy.

Found by Coverity.  This is not a problem in practice, but the results
are technically (maybe) undefined.  So make Coverity happy.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Corey Minyard <cminyard@mvista.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>