BACnet: fix dissection of exception-schedule property.

The exception schedule special-event disection already used matched
open-close tags and only needed to check for an unmatched close tag
to exit when used with ReadPropertyMultiple service.

Bug: 10691
Change-Id: I54f2f6f3f470138a6a88f84c62fd15b07ea74c37
Reviewed-on: https://code.wireshark.org/review/5593
Petri-Dish: Alexis La Goutte <alexis.lagoutte@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>