osx: fix crash when writing nicknames/professions
osx: fix crash when writing nicknames/professions

 * This patch initializes the m_alloc_start and m_alloc_end variables.
   write_string() would get bogus addresses from alloc_chunk() due to them
   not being initialized.

jarmar/Dwarf-Therapist,splintermind/Dwarf-Therapist,splintermind/Dwarf-Therapist,jarmar/Dwarf-Therapist,splintermind/Dwarf-Therapist,jarmar/Dwarf-Therapist,jarmar/Dwarf-Therapist,splintermind/Dwarf-Therapist