fix(api): open_win leak from naughty autocommands

Problem: error set by win_set_buf may leak if autocommands immediately close the
new window.

Solution: free the error set by win_set_buf. (prefer nvim_open_win's error as
it's more important and will cause 0 to be returned)

src/nvim/api/win_config.c

@@ -337,6 +337,7 @@ Window nvim_open_win(Buffer buffer, Boolean enter, Dict(win_config) *config, Err
     }
   }
   if (!tp) {
+    api_clear_error(err);  // may have been set by win_set_buf
     api_set_error(err, kErrorTypeException, "Window was closed immediately");
     goto cleanup;
   }