rnsap: fix use-after-free of "obj_id"

rnsap: fix use-after-free of "obj_id"

dissect_PrivateIEFieldValue could use "obj_id" after it was freed. Use
per-packet info instead of globals to avoid such dangling pointers and
erase any previous state to avoid interference in the same packet.

Change-Id: I7376210ef02a8e781b5a34858ae47d2254c74948
Link: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=4311
Reviewed-on: https://code.wireshark.org/review/27650
Petri-Dish: Peter Wu <peter@lekensteyn.nl>
Tested-by: Petri Dish Buildbot
Reviewed-by: Darien Spencer <cusneud@mail.com>
Reviewed-by: Peter Wu <peter@lekensteyn.nl>