This adds some more null checks to avoid a relational comparison with a
null pointer, similar to 78198b6021a9695054dab039340202170b88423c.

libstdc++-v3/ChangeLog:

	PR libstdc++/97415
	* include/std/sstream (basic_stringbuf::_M_update_egptr)
	(basic_stringbuf::__xfer_bufptrs::__xfer_bufptrs): Check for
	null before comparing pointers.

Tested powerpc64le-linux. Committed to trunk.