The C++ char_traits and ctype APIs do not disallow null pointer
arguments, so we need explicit checks to ensure we don't forward null
pointers to memcpy or memmove.

	PR libstdc++/87787
	* include/bits/char_traits.h (char_traits::move): Do not pass null
	pointers to memmove.
	* include/bits/locale_facets.h
	(ctype<char>::widen(const char*, const char*, char*)): Do not
	pass null pointers to memcpy.
	(ctype<char>::narrow(const char*, const char*, char, char*)):
	Likewise.
	(ctype<char>::do_widen(const char*, const char*, char*)):
	Likewise.
	(ctype<char>::do_narrow(const char*, const char*, char, char*)):
	Likewise.

Tested powerpc64-linux, committed to trunk.