CPP13 — Exceptions in servant constructors

I think we have a defect/omission in the C++ mapping with respect
to exception safety. Consider a servant class with a constructor:

class FooImpl : public virtual POA_Foo

;

Consider what happens if FooImpl() throws an exception for some reason.
By the time FooImpl() runs, the base class constructor POA_Foo() has run
already. So, when the compiler deals with the exception, it invokes
~POA_Foo().

The problem arises because, in our implementation at least, ~POA_Foo()
checks if the reference count is zero and asserts if not.