Check explicitly for mark being set when trying to partition, as we shouldn't be messing with the x address, etc.

Original nano commit: 4585
Original nano commit message:
 	2014-01-24 Benno Schulenberg <bens>
        * src/nano.c (copy_from_filestruct) - Check explicitly for mark being set when
          trying to partition, as we shouldn't be messing with the x address, etc.