Handle empty interface lists when the list changes.

Don't assume that a change to the interface list is from a non-empty
interface list to a different non-empty interface list.

Change-Id: I94054ca0cf2661704aff1869385aa2155c19677d
Reviewed-on: https://code.wireshark.org/review/3193
Reviewed-by: Guy Harris <guy@alum.mit.edu>