Fix leaks when spawning extcap processes

Fix leaks when spawning extcap processes

Valgrind reports leaks like these when spawning extcap processes (for
example when displaying or refreshing interface list with extcap
interfaces present):

3,917 (464 direct, 3,453 indirect) bytes in 29 blocks are definitely lost in loss record 58,301 of 58,638
   at 0x4C2AB80: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
   by 0xA6D2610: g_malloc (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.4002.0)
   by 0xA6E822D: g_slice_alloc (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.4002.0)
   by 0xA6B913B: g_error_new_valist (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.4002.0)
   by 0xA6B94ED: g_set_error (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.4002.0)
   by 0xA710B03: ??? (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.4002.0)
   by 0xA710E17: g_spawn_sync (in /lib/x86_64-linux-gnu/libglib-2.0.so.0.4002.0)
   by 0x44C677: extcap_foreach (extcap.c:199)
   by 0x44CCAD: extcap_interface_list (extcap.c:413)
   by 0x72B548: capture_interface_list (capture_ifinfo.c:126)
   by 0x7336FA: scan_local_interfaces (iface_lists.c:141)
   by 0x55F01F: WiresharkApplication::refreshLocalInterfaces() (wireshark_application.cpp:898)
   ...

Change-Id: If8f750f5f8fa42a6f0884bb0e6bbbd71bd8f68aa
Reviewed-on: https://code.wireshark.org/review/13631
Petri-Dish: Roland Knall <rknall@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Roland Knall <rknall@gmail.com>