Don't free global_capture_opts.ifaces_err_info.

Leave it around in case InterfaceTree::display() gets invoked without an
intervening call to scan_local_interfaces() (that would cause a
double-free) or in case scan_local_interfaces() gets called afterwards
(as it frees global_capture_opts.ifaces_err_info before setting it
again, so there's no leak there, but there would be a double-free).

This should prevent at least one crash I've seen.

Change-Id: Iedd3959b420c09971557c473cc87aefd4454078c
Reviewed-on: https://code.wireshark.org/review/4917
Reviewed-by: Guy Harris <guy@alum.mit.edu>