Prevent crashes when embedding Emacs in a nonexistent parent

Prevent crashes when embedding Emacs in a nonexistent parent

* src/xfns.c (x_window, Fx_create_frame): Handle X errors while
trying to reparent the frame onto an explicitly specified
parent.
* src/xterm.c (handle_one_xevent): Handle unparenting embedded
windows correctly.  This only works if the embedder is aware of
the fixes extension and has put the window into the right save
set.
(x_embed_frame): New function.
* src/xterm.h (FRAME_X_EMBEDDED_P): Fix coding style.