Qt: fix a potential memory leak FollowStreamDialog::follow() found by Coverity (CID 1159304)

Change-Id: I53709c6729c6d9156d2ba5ccd83e92bc27433e26
Reviewed-on: https://code.wireshark.org/review/5065
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>
Reviewed-by: Anders Broman <a.broman58@gmail.com>