Make "xfree (NULL)" a no-op; remove useless if-before-xfree.

Make "xfree (NULL)" a no-op; remove useless if-before-xfree.

* src/alloc.c (xfree): Return right away for a NULL arg.
* src/lread.c (nosuffix): Remove now-useless if-before-xfree tests.
* src/gtkutil.c (xg_gtk_scroll_destroy): Likewise.
* src/mac.c (create_apple_event_from_event_ref): Likewise.
(create_apple_event_from_drag_ref, cfstring_create_normalized): Likewise.
* src/doprnt.c (doprnt1): Likewise.
* src/frame.c (frame): Likewise.
* src/keyboard.c (wipe_kboard): Likewise.
* src/macterm.c (x_free_frame_resources, xlfdpat_destroy, XFreePixmap):
(init_font_name_table, mac_unload_font, x_delete_display): Likewise.
* src/term.c (tty_default_color_capabilities, maybe_fatal)
(delete_tty): Likewise.
* src/w16select.c (string): Likewise.
* src/w32.c (w32_get_resource, SET_ENV_BUF_SIZE): Likewise.
* src/w32bdf.c (w32_free_bdf_font): Likewise.
* src/w32fns.c (w32_unload_font): Likewise.
* src/w32font.c (w32font_close): Likewise.
* src/window.c (size_window): Likewise.
* src/xselect.c (receive_incremental_selection): Likewise.
* src/xterm.c (x_free_frame_resources, x_delete_display): Likewise.
* src/mactoolbox.c (create_apple_event_from_drag_ref): Likewise.
* src/w32.c (stat): Likewise.