qtest.c: Allow zero size in memset qtest commands

Some tests use the qtest protocol "memset" command with a zero
size, expecting it to do nothing. However in the current code this
will result in calling memset() with a NULL pointer, which is
undefined behaviour. Detect and specially handle zero sizes to
avoid this.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1470393800-7882-1-git-send-email-peter.maydell@linaro.org