apply patch from DB: in write_file(), if we've tried to write to an

apply patch from DB: in write_file(), if we've tried to write to an
unwritable file and we're not prepending, tempname is NULL when it's
passed to unlink(); this can cause problems if unlink() can't handle
NULL, so don't call it in that case

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@2012 35c25a1d-7b9e-4130-9fde-d3aeb78583b8