vim-patch:8.2.1106: crash when trying to use s: variable in typed command

Problem:    Crash when trying to use s: variable in typed command.
Solution:   Don't use the script index when not set. (Ken Takata,
            closes vim/vim#6366)

8e6cbb7232

src/nvim/testdir/test_vimscript.vim

@@ -1810,6 +1810,7 @@ func Test_float_conversion_errors()
   endif
 endfunc
 
+" invalid function names               {{{1
 func Test_invalid_function_names()
   " function name not starting with capital
   let caught_e128 = 0
@@ -1870,7 +1871,7 @@ func Test_invalid_function_names()
   call delete('Xscript')
 endfunc
 
-" substring and variable name
+" substring and variable name              {{{1
 func Test_substring_var()
   let str = 'abcdef'
   let n = 3
@@ -1890,6 +1891,20 @@ func Test_substring_var()
   unlet b:nn
 endfunc
 
+" Test using s: with a typed command              {{{1
+func Test_typed_script_var()
+  CheckRunVimInTerminal
+
+  let buf = RunVimInTerminal('', {'rows': 6})
+
+  " Deep nesting of if ... endif
+  call term_sendkeys(buf, ":echo get(s:, 'foo', 'x')\n")
+  call TermWait(buf)
+  call WaitForAssert({-> assert_match('^E116:', term_getline(buf, 5))})
+
+  call StopVimInTerminal(buf)
+endfunc
+
 func Test_for_over_string()
   let res = ''
   for c in 'aéc̀d'