lib/message.cc: fix Coverity finding (use after free)

The object where pointer to `data` was received was deleted before
it was used in _notmuch_string_list_append().

Relevant Coverity messages follow:

3: extract
Assigning: data = std::__cxx11::string(message->doc.()).c_str(),
which extracts wrapped state from temporary of type std::__cxx11::string.

4: dtor_free
The internal representation of temporary of type std::__cxx11::string
is freed by its destructor.

5: use after free:
Wrapper object use after free (WRAPPER_ESCAPE)
Using internal representation of destroyed object local data.