c++ question regarding exception safety

In Section E.3.5. of the book 'the c++ programming language 3rd
edtion' by Bjarne Stroustrup, why it needs to add the check: 'if (v)'
in the vector_base destrctor of the vector_base implmentation.

In my understanding, if this statement failed(v=alloc.allocate(n)) in
the contrstructor, the destrctor will never be called. So no check
needed in the destructor.

But according to the book, Bjarned says clearly in the book(p. 950,
Section E.3.5)

Note that this attempt to write safer code complicates the invariant
for the class: It is no longer
guaranteed that v points to allocated memory. Now v might be 0 .

I am confused now.

Thanks for your help,

Michael