usb/dev-mtp: Fix use of uninitialized values

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 <[email protected]>
Message-id: 20180604151421[email protected]
Signed-off-by: Gerd Hoffmann <[email protected]>