libstdc++/8716: std::string( NULL, 0 ) throws exception also on zero length

[Paolo Carlini]

Yes, Phil the testcase doesn't fail as an *intended* effect of
the fix for 8347: it's nice to see the patch meeting user's
expectations for this different pattern too!

I think its worthy to add to the testsuite the new testcase.
Could you possibly do it?

Thanks, Paolo.

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=8716