hw/milkymist-softusb: set buffer in softusb_read_{dmem, pmem} error path

Make sure we set the buffer to something in the softusb_read_{dmem,pmem}
error paths, since the caller will use the buffer unconditionally.
(Newer gcc is smart enough to spot this and complain about 'may be
used uninitialized'.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1364496184-11994-1-git-send-email-peter.maydell@linaro.org
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
