vim-patch:8.2.3850: illegal memory access when displaying a partial

Problem:    Illegal memory access when displaying a partial.
Solution:   Terminate the string with a NUL. (closes vim/vim#9371)
2de5371a75

src/nvim/testdir/test_messages.vim

@@ -108,3 +108,11 @@ func Test_echospace()
 
   set ruler& showcmd&
 endfunc
+
+" this was missing a terminating NUL
+func Test_echo_string_partial()
+  function CountSpaces()
+  endfunction
+  echomsg function('CountSpaces', [#{aaaaaaaaaaa: v:false, bbbbbbbbbbbb: '', ccccccccccc: ['ab', 'cd']}])
+endfunc
+