Pacify GCC 10.1.0

Pacify GCC 10.1.0

Pacify GCC 10.1.0 so that it does not issue false alarms
when Emacs is configured with --enable-gcc-warnings.
* src/dispnew.c (clear_glyph_row):
* src/fns.c (hash_clear):
* src/keyboard.c (append_tab_bar_item):
* src/lisp.h (vcopy):
* src/xfaces.c (get_lface_attributes_no_remap)
(Finternal_copy_lisp_face, realize_default_face):
* src/xmenu.c (set_frame_menubar):
Work around -Warray-bounds false alarm in GCC 10.1.0.
* src/intervals.c (copy_properties):
Avoid -Wnull-dereference false alarm in GCC 10.1.0.
* src/lisp.h (xvector_contents_addr, xvector_contents):
New functions, useful for working around GCC bug 95072.