[Bug libstdc++/47913] [C++0x] improve ratio_add to overflow less often

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47913

--- Comment #16 from Paolo Carlini <paolo.carlini at oracle dot com> 2011-03-02 
14:59:23 UTC ---
Done. Then we can add more tests to 47913.cc.