Fix appending the list of remote interfaces.

Fix appending the list of remote interfaces.

Don't assume if_list is non-null when append_remote_list() is called; it
won't be, if we didn't find any local interfaces.  Have
append_remote_list() return the final if_list value, and assign it to
if_list.

Change-Id: I05586d5b034f3d61aa38a20f6c6d2c69117b367e
Reviewed-on: https://code.wireshark.org/review/14191
Petri-Dish: Guy Harris <guy@alum.mit.edu>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Guy Harris <guy@alum.mit.edu>