vim-patch:9.0.2126: unused assignments when checking 'listchars' (#26182)

Problem:  Unused assignments when checking the value of 'listchars'.
Solution: Loop only once when just checking the value.  Add a test to
          check that this change doesn't cause double-free.

closes: vim/vim#13559

00624a2fa0

test/old/testdir/test_listchars.vim

@@ -265,6 +265,11 @@ func Test_listchars()
   call Check_listchars(expected, 5, 12)
   call assert_equal(expected, split(execute("%list"), "\n"))
 
+  " Changing the value of 'ambiwidth' twice shouldn't cause double-free when
+  " "leadmultispace" is specified.
+  set ambiwidth=double
+  set ambiwidth&
+
   " Test leadmultispace and lead and space
   normal ggdG
   set listchars=eol:$  " Accommodate Nvim default