Crash with invalid use of boost::optional<fontset>

[springmeyer]

Thread 7 Crashed:
0   libmapnik.dylib                 0x000000010538b49c boost::optional_detail::optional_base<mapnik::font_set>::is_initialized() const + 12
1   libmapnik.dylib                 0x000000010538b561 boost::optional<mapnik::font_set>::operator!() const + 17
2   libmapnik.dylib                 0x000000010550bc5c mapnik::map_parser::parse_text_symbolizer(mapnik::rule&, mapnik::xml_node const&) + 1772
3   libmapnik.dylib                 0x0000000105506f06 mapnik::map_parser::parse_symbolizers(mapnik::rule&, mapnik::xml_node const&) + 1302
4   libmapnik.dylib                 0x00000001055058f4 mapnik::map_parser::parse_rule(mapnik::feature_type_style&, mapnik::xml_node const&) + 3476
5   libmapnik.dylib                 0x00000001054fe715 mapnik::map_parser::parse_style(mapnik::Map&, mapnik::xml_node const&) + 6181
6   libmapnik.dylib                 0x00000001054fb12c mapnik::map_parser::parse_map_include(mapnik::Map&, mapnik::xml_node const&) + 940
7   libmapnik.dylib                 0x00000001054fa6d0 mapnik::map_parser::parse_map(mapnik::Map&, mapnik::xml_node const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&) + 14768
8   libmapnik.dylib                 0x00000001054faa78 mapnik::load_map_string(mapnik::Map&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, bool, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >) + 744
9   mapnik.node                     0x00000001039eaaf1 Map::EIO_FromString(uv_work_s*) + 129 (mapnik_map.cpp:1200)
10  node                            0x0000000100447786 worker + 90
11  node                            0x000000010043dd9a uv__thread_start + 25
12  libsystem_pthread.dylib         0x00007fff86338268 _pthread_body + 131
13  libsystem_pthread.dylib         0x00007fff863381e5 _pthread_start + 176
14  libsystem_pthread.dylib         0x00007fff8633641d thread_start + 13