reformat dissect_usb_setup_response()
make sure it can handle usb_conv_info==NULL

Change-Id: I5f53b26fd6fe22471906b07b5dad8f4c07dffa55
Reviewed-on: https://code.wireshark.org/review/3843
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>