Optimize a mark-restoring condition after document reload

The loop that restores marks compares the index of a temporary mark `z`
to the number of lines in the document `lines`. The check virtually
always succeeds and makes no sense. Compare the temporary mark's line
number to the number of lines instead. When the temporary mark's line
number is out of the document range, setMark() does nothing, so this
commit is only an optimization.