patch 8.0.0685: when conversion fails written file may be truncated

Problem:    When making backups is disabled and conversion with iconv fails
            the written file is truncated. (Luo Chen)
Solution:   First try converting the file and write the file only when it did
            not fail. (partly by Christian Brabandt)

src/testdir/test_writefile.vim

@@ -31,3 +31,21 @@ func Test_writefile_fails_gently()
 
   call assert_fails('call writefile([], [])', 'E730:')
 endfunc
+
+func Test_writefile_fails_conversion()
+  if !has('multi_byte') || !has('iconv')
+    return
+  endif
+  set nobackup nowritebackup
+  new
+  let contents = ["line one", "line two"]
+  call writefile(contents, 'Xfile')
+  edit Xfile
+  call setline(1, ["first line", "cannot convert \u010b", "third line"])
+  call assert_fails('write ++enc=cp932')
+  call assert_equal(contents, readfile('Xfile'))
+
+  call delete('Xfile')
+  bwipe!
+  set backup& writebackup&
+endfunc