warning: saved_conditional may be used uninitialized

[alandefreitas]

We get this warning every time we build the docs:

    inlined from ‘void quickbook::quickbook_grammar::impl::init_phrase_elements()’ at /home/alandefreitas/Documents/Code/C++/boost/tools/quickbook/src/phrase_element_grammar.cpp:41:65:
/home/alandefreitas/Documents/Code/C++/boost/tools/quickbook/src/actions.hpp:184:12: warning: ‘<unnamed>.quickbook::cond_phrase_push::saved_conditional’ may be used uninitialized [-Wmaybe-uninitialized]
  184 |     struct cond_phrase_push : scoped_action_base
      |            ^~~~~~~~~~~~~~~~