SPOOLSS: Try to avoid an infinite loop.

Use tvb_reported_length_remaining in dissect_spoolss_uint16uni. Make
sure our offset always increments in dissect_spoolss_keybuffer.

Change-Id: I7017c9685bb2fa27161d80a03b8fca4ef630e793
Reviewed-on: https://code.wireshark.org/review/14687
Reviewed-by: Gerald Combs <gerald@wireshark.org>
Petri-Dish: Gerald Combs <gerald@wireshark.org>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>