* lisp.h (XCONS, XSTRING, XSYMBOL, XFLOAT, XPROCESS, XWINDOW, XSUBR, XBUFFER):

* lisp.h (XCONS, XSTRING, XSYMBOL, XFLOAT, XPROCESS, XWINDOW, XSUBR, XBUFFER):
Verify correct object type before returning pointer, using eassert.
* frame.h (XFRAME): Likewise.

* buffer.c (Frename_buffer, Fset_buffer_multibyte,
swap_out_buffer_local_variables, Fmove_overlay): Don't apply XSYMBOL, XBUFFER,
etc, to values that may be nil or of the wrong type.
* data.c (set_internal): Likewise.
* dispextern.h (WINDOW_WANTS_MODELINE_P, WINDOW_WANTS_HEADER_LINE_P): Likewise.
* fileio.c (auto_save_1): Likewise.
* insdel.c (check_markers): Likewise.
* marker.c (buf_charpos_to_bytepos, unchain_marker): Likewise.
* undo.c (record_insert): Likewise.
* vmsproc.c (child_sig): Likewise.
* window.c (unshow_buffer, window_loop): Likewise.
* xterm.c (x_erase_phys_cursor): Likewise.