vim-patch:8.2.4436: crash with weird 'vartabstop' value

Problem:    Crash with weird 'vartabstop' value.
Solution:   Check for running into the end of the line.

4e889f98e9

Code change is N/A as it's superseded by virtual text changes.

Co-authored-by: Bram Moolenaar <Bram@vim.org>

test/old/testdir/test_vartabs.vim

@@ -445,4 +445,16 @@ func Test_shiftwidth_vartabstop()
   setlocal shiftwidth& vartabstop& tabstop&
 endfunc
 
+func Test_vartabstop_latin1()
+  let save_encoding = &encoding
+  new
+  set encoding=iso8859
+  silent norm :se 

+  set vartabstop=400
+  norm i00	
+  bwipe!
+  let &encoding = save_encoding
+endfunc
+
+
 " vim: shiftwidth=2 sts=2 expandtab