USB HID descriptors may push/pop the current state to allow

USB HID descriptors may push/pop the current state to allow
description of items residing in a so-called union. FreeBSD currently
only supports 4 such pop levels.

If the push level is not restored within the processing of the same
HID item, an invalid memory location may be used for subsequent HID
item processing.

Verify that the push level is always valid when processing HID items.

Reported by:	Andy Nguyen (Google)
MFC after:	3 days
Sponsored by:	Mellanox Technologies