patch 8.2.3669: buffer overflow with long help argument

Problem:    Buffer overflow with long help argument.
Solution:   Use snprintf().

src/testdir/test_help.vim

@@ -134,4 +134,13 @@ func Test_help_window_height()
   close
 endfunc
 
+func Test_help_long_argument()
+  try
+    exe 'help \%' .. repeat('0', 1021)
+  catch
+    call assert_match("E149:", v:exception)
+  endtry
+endfunc
+
+
 " vim: shiftwidth=2 sts=2 expandtab