Prevent double free in basic_string

[Paolo Carlini]

.. at this time in my opinion we should not fiddle with delicate 
details of the reference-counted implementation, it should be considered 
largely frozen besides fixing regressions (eg, caused by stronger 
optimizations) and experimentally implementing C++11 features (the 
latter because we started to do that consistently with the containers, I 
wasn't sure, at that time). This, as a matter of general policy, even if 
the patch were in principle correct.

Paolo.