libstdc++: Do not set eofbit eagerly in operator>>(istream&, char(&)[N])

libstdc++: Do not set eofbit eagerly in operator>>(istream&, char(&)[N])

Similar to the bugs I fixed recently in istream::ignore, we incorrectly
set eofbit too often in operator>>(istream&, string&) and
operator>>(istream&.  char(&)[N]).

We should only set eofbit if we reach EOF but would have kept going
otherwise. If we've already extracted the maximum number of characters
(whether that's because of the buffer size or the istream's width())
then we should not set eofbit.

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.tcc
	(operator>>(basic_istream&, basic_string&)): Do not set eofbit
	if extraction stopped after in.width() characters.
	* src/c++98/istream-string.cc (operator>>(istream&, string&)):
	Likewise.
	* include/bits/istream.tcc (__istream_extract): Do not set
	eofbit if extraction stopped after n-1 characters.
	* src/c++98/istream.cc (__istream_extract): Likewise.
	* testsuite/21_strings/basic_string/inserters_extractors/char/13.cc: New test.
	* testsuite/21_strings/basic_string/inserters_extractors/wchar_t/13.cc: New test.
	* testsuite/27_io/basic_istream/extractors_character/char/5.cc: New test.
	* testsuite/27_io/basic_istream/extractors_character/wchar_t/5.cc: New test.