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>

hw/ipmi/smbus_ipmi.c

@@ -281,7 +281,9 @@ static int ipmi_write_data(SMBusDevice *dev, uint8_t *buf, uint8_t len)
              */
             send = true;
         }
+        if (len > 0) {
             memcpy(sid->inmsg + sid->inlen, buf, len);
+        }
         sid->inlen += len;
         break;
     }