PR libstdc++/79820
	PR libstdc++/81751
	* config/io/basic_file_stdio.cc (sys_open(FILE*, ios_base::openmode)):
	Call fflush on the stream instead of calling sync() while _M_cfile is
	null. Restore original value of errno.
	* testsuite/ext/stdio_filebuf/char/79820.cc: New.
	* testsuite/ext/stdio_filebuf/char/81751.cc: New.

From-SVN: r250993