fix(autocmd): heap UAF with :bwipe in Syntax autocmd

Problem:
Creating an autocommand which executes `:bwipe` on the Syntax event
causes a heap-use-after-free.

Solution: set BF_SYN_SET flag before applying autocommands
(cherry picked from commit f36dd7557a)

src/nvim/option.c

@@ -2726,11 +2726,11 @@ static void do_syntax_autocmd(buf_T *buf, bool value_changed)
   static int syn_recursive = 0;
 
   syn_recursive++;
+  buf->b_flags |= BF_SYN_SET;
   // Only pass true for "force" when the value changed or not used
   // recursively, to avoid endless recurrence.
   apply_autocmds(EVENT_SYNTAX, buf->b_p_syn, buf->b_fname,
                  value_changed || syn_recursive == 1, buf);
-  buf->b_flags |= BF_SYN_SET;
   syn_recursive--;
 }
 

test/functional/autocmd/autocmd_spec.lua

@@ -716,6 +716,15 @@ describe('autocmd', function()
     ]]
   end)
 
+  it('no use-after-free when wiping buffer in Syntax autocommand', function()
+    exec([[
+      new
+      autocmd Syntax * ++once bwipe!
+      setlocal syntax=vim
+    ]])
+    assert_alive()
+  end)
+
   it('no use-after-free from win_enter autocommands in win_move_after', function()
     exec [[
       split foo