we either have a source or a destination file name
when we set one, we clear the other

Change-Id: I7c316dc9cc7b2c93cfa1b00eb617553f55a58715
Reviewed-on: https://code.wireshark.org/review/3449
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>