extcap: Fix misc memory leaks triggered by network interface changes

extcap: Fix misc memory leaks triggered by network interface changes

Valgrind reports plenty of misc memory leaks in extcap after the network
interface list has changed or is refreshed. Errors can be seen by
starting Wireshark with Valgrind's memcheck tool and bringing a network
interface up and down a few times with:

ifconfig eth0 up
ifconfig eth0 down

Change-Id: I90f53847071854b7d02facb39b7a380732de79b4
Reviewed-on: https://code.wireshark.org/review/17606
Petri-Dish: Peter Wu <peter@lekensteyn.nl>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Peter Wu <peter@lekensteyn.nl>