patch 8.2.4899: with latin1 encoding CTRL-W might go before the cmdline

Problem:    With latin1 encoding CTRL-W might go before the start of the
            command line.
Solution:   Check already being at the start of the command line.
diff --git a/src/testdir/test_cmdline.vim b/src/testdir/test_cmdline.vim
index 474638f..5a849f7 100644
--- a/src/testdir/test_cmdline.vim
+++ b/src/testdir/test_cmdline.vim
@@ -773,6 +773,9 @@
 
     call feedkeys(":abc def\<S-Left>\<C-U>\<C-B>\"\<CR>", 'tx')
     call assert_equal('"def', @:, e)
+
+    " This was going before the start in latin1.
+    call feedkeys(": \<C-W>\<CR>", 'tx')
   endfor
 
   let &encoding = encoding_save