usb/dev-mtp: Fix use of uninitialized values
    
    This fixes:
    
      hw/usb/dev-mtp.c:971:5: warning: 4th function call argument is an uninitialized value
          trace_usb_mtp_op_get_partial_object(s->dev.addr, o->handle, o->path,
                                               c->argv[1], c->argv[2]);
                                                           ^~~~~~~~~~
    and:
    
      hw/usb/dev-mtp.c:981:12: warning: Assigned value is garbage or undefined
          offset = c->argv[1];
                   ^ ~~~~~~~~~~
    
    Reported-by: Clang Static Analyzer
    Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
    Message-id: 20180604151421.23385-3-f4bug@amsat.org
    Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
    (cherry picked from commit 62713a2e50f653162387451034f1a2490e87be88)
    Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>