fstream.tcc (basic_filebuf<>::underflow): Don't call codecvt::in at all if _M_ext_next >= _M_ext_end.

2007-03-27  Paolo Carlini  <pcarlini@suse.de>

	* include/bits/fstream.tcc (basic_filebuf<>::underflow):
	Don't call codecvt::in at all if _M_ext_next >= _M_ext_end.
	* testsuite/27_io/basic_filebuf/underflow/char/3.cc: New.

From-SVN: r123260