Fix crash found by vttest.

If DL was passed a parameter equal to or greater than the number of
lines from the current line to the last line in the region, it would
try to move a negative-sized box up, which resulted in a crash.

This fixes that three ways:
1) Do not allow moving boxes with negative X/Y sizes
2) In dellines, clamp the lines to the number that could be deleted
3) Don't move lines up of all rest of the lines are being deleted