Fix memleaks related to get_dirname

Fix memleaks related to get_dirname

get_dirname may return NULL instead of the original string, so avoid
patterns like get_dirname(strdup(x)). Writing to
cf_path.toUtf8().data() is fine btw, toUtf8() returns new memory.

This fixes two memleak reported by LeakSanitizer via fileset_add_dir and
MainWindow::captureFileReadFinished (both via cf_callback_invoke).

Change-Id: I0f1528763e77e1f55b54b6674c890a9d02302ee8
Reviewed-on: https://code.wireshark.org/review/13691
Petri-Dish: Dario Lombardo <lomato@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>