libstdc++: Adjust overflow prevention to operator>>

libstdc++: Adjust overflow prevention to operator>>

This adjusts the overflow prevention added to operator>> so that we can
distinguish "unknown size" from "zero size", and avoid writing anything
at all in to zero sized buffers.

This also removes the incorrect comment saying extraction stops at a
null byte.

libstdc++-v3/ChangeLog:

	* include/std/istream (operator>>(istream&, char*)): Add
	attributes to get warnings for pointers that are null or known
	to point to the end of a buffer. Request upper bound from
	__builtin_object_size check and handle zero-sized buffer case.
	(operator>>(istream&, signed char))
	(operator>>(istream&, unsigned char*)): Add attributes.
	* testsuite/27_io/basic_istream/extractors_character/char/overflow.cc:
	Check extracting into the middle of a buffer.
	* testsuite/27_io/basic_istream/extractors_character/wchar_t/overflow.cc: New test.