usb: correctly handle Zero Length Packets

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>