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.

src/nvim/api/vim.c

@@ -967,13 +967,15 @@ Buffer nvim_create_buf(Boolean listed, Boolean scratch, Error *err)
 
   // Open the memline for the buffer. This will avoid spurious autocmds when
   // a later nvim_buf_set_lines call would have needed to "open" the buffer.
-  try_start();
-  block_autocmds();
-  int status = ml_open(buf);
-  unblock_autocmds();
-  try_end(err);
-  if (status == FAIL) {
-    goto fail;
+  if (buf->b_ml.ml_mfp == NULL) {
+    try_start();
+    block_autocmds();
+    int status = ml_open(buf);
+    unblock_autocmds();
+    try_end(err);
+    if (status == FAIL) {
+      goto fail;
+    }
   }
 
   // Set last_changedtick to avoid triggering a TextChanged autocommand right

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()