vim-patch:9.0.2141: [security]: buffer-overflow in suggest_trie_walk

Problem:  [security]: buffer-overflow in suggest_trie_walk
Solution: Check n before using it as index into byts array

Basically, n as an index into the byts array, can point to beyond the byts
array. So let's double check, that n is within the expected range after
incrementing it from sp->ts_curi and bail out if it would be invalid.

Reported by @henices, thanks!

0fb375aae6

Co-authored-by: Christian Brabandt <cb@256bit.org>

test/old/testdir/test_crash.vim

@@ -135,6 +135,13 @@ func Test_crash1_2()
     \ '  && echo "crash 2: [OK]" >> '.. result .. "\<cr>")
   call TermWait(buf, 350)
 
+  let file = 'crash/poc_suggest_trie_walk'
+  let cmn_args = "%s -u NONE -i NONE -n -e -s -S %s -c ':qa!'"
+  let args = printf(cmn_args, vim, file)
+  call term_sendkeys(buf, args ..
+    \ '  && echo "crash 3: [OK]" >> '.. result .. "\<cr>")
+  call TermWait(buf, 150)
+
   " clean up
   exe buf .. "bw!"
 
@@ -143,6 +150,7 @@ func Test_crash1_2()
   let expected = [
       \ 'crash 1: [OK]',
       \ 'crash 2: [OK]',
+      \ 'crash 3: [OK]',
       \ ]
 
   call assert_equal(expected, getline(1, '$'))