Don’t err in pip-frame-remove-buffer if there is no PIP frame

This is useful e.g. to prevent the error when a PIP frame with a
temporarily displayed buffer is deleted before the buffer timer
attempts to remove the buffer.

pip-frame.el

@@ -175,7 +175,8 @@ If the buffer is not present in the PIP frame, do nothing."
   (interactive (list (completing-read "Remove PIP buffer: "
                                       (mapcar #'buffer-name (pip-frame--buffers))
                                       nil t)))
-  (let* ((windows (window-list (pip-frame--get-frame)))
+  (if-let ((frame (pip-frame--get-frame t))
+           (windows (window-list frame))
            (buffer (get-buffer buffer-or-name))
            (windows-to-delete (cl-remove buffer windows
                                          :key #'window-buffer