fix breakage in both backup_lines() and do_justify(); the latter's

fix breakage in both backup_lines() and do_justify(); the latter's
return value is unneeded, as current will point to the same line when
it's done, and making first_par_line point to current in the former
ensures that the two will properly stay in sync

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@3155 35c25a1d-7b9e-4130-9fde-d3aeb78583b8