fix(api): nvim_win_set_buf(0, 0) fails if 'winfixbuf' is set #31576

## Problem
With 'winfixbuf' enabled, `nvim_win_set_buf` and `nvim_set_current_buf` fail
even if targeting the already-current buffer.

    vim.wo.winfixbuf = true
    vim.api.nvim_win_set_buf(0, 0)
    vim.api.nvim_set_current_buf(0)

Solution:
Check for this condition.

src/nvim/api/window.c

@@ -59,7 +59,7 @@ void nvim_win_set_buf(Window window, Buffer buffer, Error *err)
 {
   win_T *win = find_window_by_handle(window, err);
   buf_T *buf = find_buffer_by_handle(buffer, err);
-  if (!win || !buf) {
+  if (!win || !buf || win->w_buffer == buf) {
     return;
   }