fix(man.lua): show_toc condition may cause infinite loop #36979

`lnum` gets set with `vim.fn.nextnonblank`, which returns 0 on failure,
and which is truthy on Lua.

(cherry picked from commit 46220afef8)

runtime/lua/man.lua

@@ -795,7 +795,7 @@ function M.show_toc()
 
   local lnum = 2
   local last_line = fn.line('$') - 1
-  while lnum and lnum < last_line do
+  while lnum > 0 and lnum < last_line do
     local text = fn.getline(lnum)
     if text:match('^%s+[-+]%S') or text:match('^   %S') or text:match('^%S') then
       toc[#toc + 1] = {