libstdc++: Fix more unspecified comparisons to null pointer [PR 97415]

This adds some more null checks to avoid a relational comparison with a
null pointer, similar to 78198b60.

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.