usb: correctly handle Zero Length Packets
    
    USB Specification Revision 2.0, §5.5.3:
      The Data stage of a control transfer from an endpoint to the host is complete when the endpoint does one of the following:
      • Has transferred exactly the amount of data specified during the Setup stage
      • Transfers a packet with a payload size less than wMaxPacketSize or transfers a zero-length packet"
    
    hw/usb/redirect.c:802:9: warning: Declared variable-length array (VLA) has zero size
            uint8_t buf[size];
            ^~~~~~~~~~~ ~~~~
    
    Reported-by: Clang Static Analyzer
    Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
    Message-id: 20180604151421.23385-2-f4bug@amsat.org
    Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
    (cherry picked from commit bf78fb1c1b61a819a47f7a1dbecf9934b9f32a0d)
    Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>