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

The standard doesn't guarantee that null pointers compare less than
non-null pointers. AddressSanitizer complains about the pptr()> egptr()
comparison in basic_stringbuf::str() when egptr() is null.

libstdc++-v3/ChangeLog:

	PR libstdc++/97415
	* include/std/sstream (basic_stringbuf::str()): Check for
	null egptr() before comparing to non-null pptr().