fix(api): nvim_create_buf leaks memory if buffer is loaded early

Problem: memory leak in nvim_create_buf if buflist_new autocommands load the
new buffer early.

Solution: do not open a memfile in that case.

test/functional/api/vim_spec.lua

@@ -3135,6 +3135,15 @@ describe('API', function()
       -- nowadays this works because we don't execute any spurious autocmds at all #24824
       assert_alive()
     end)
+
+    it('no memory leak when autocommands load the buffer immediately', function()
+      exec([[
+        autocmd BufNew * ++once call bufload(expand("<abuf>")->str2nr())
+                             \| let loaded = bufloaded(expand("<abuf>")->str2nr())
+      ]])
+      api.nvim_create_buf(false, true)
+      eq(1, eval('g:loaded'))
+    end)
   end)
 
   describe('nvim_get_runtime_file', function()