VIDEO class descriptor walk consumes device-controlled wTotalLength as its bound

A VideoStreaming interface reports the size of its whole class-specific descriptor block in the `wTotalLength` field of its input header. The USBX host recorded that figure and used it as the budget for walking the format, frame and frame-interval descriptors, without holding it against the configuration descriptor actually received. A device that reports a `wTotalLength` larger than the descriptors it supplies therefore had its own claim believed. Once the real bytes ran out the walk kept going, dereferencing the cursor past the end of the configuration buffer, in the host's context, whenever the application queried format or frame data.