Don't free the hash if another one exists

Other dissectors may still have references. This is a quick hacky fix that adds
a (very small) memory leak since the real fix in master is way too big to
backport.

Conflicts:
	epan/dissectors/packet-rtp.c

Bug:9920
Change-Id: I6501b0e8a9616b0ddc72c4e7a7f9a9ebe145b7c5
Reviewed-on: https://code.wireshark.org/review/3342
Reviewed-by: Michael Mann <mmann78@netscape.net>
Reviewed-on: https://code.wireshark.org/review/4422
Reviewed-by: Balint Reczey <balint@balintreczey.hu>