Extcap: prevent a GLib assert when interface_opts.extcap_args is NULL

I was able to trigger it when trying to change the options of extcap_example.py interface

Change-Id: I9ecff51d47b3ef6e178637b31d49af46a1928acd
Reviewed-on: https://code.wireshark.org/review/3822
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>