tn5220: prevent another potential endless loop
exit the loop if dissect_unknown_data() returns 0

Change-Id: I681d170d4daf8efb06231790652ff7c941b46d89
Reviewed-on: https://code.wireshark.org/review/4882
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
(cherry picked from commit 1463db37d9bbc9cd532afdf2817caaf8eb367831)
Reviewed-on: https://code.wireshark.org/review/5181