in safe_tempfile(), open a newly created temporary file in "r+b" mode

in safe_tempfile(), open a newly created temporary file in "r+b" mode
instead of "w+b" mode, since the latter truncates the file if it already
exists, and we don't need to do that since it's an error if the file
already exists

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@2593 35c25a1d-7b9e-4130-9fde-d3aeb78583b8