aka MSVC warning C4715: not all control paths return a value

diff -ru boost.org/boost/property_tree/json_parser/detail/standard_callbacks.hpp boost/boost/property_tree/json_parser/detail/standard_callbacks.hpp
--- foo/misc/boost.org/boost/property_tree/json_parser/detail/standard_callbacks.hpp	2015-07-07 14:20:48.000000000 +0200
+++ foo/misc/boost/boost/property_tree/json_parser/detail/standard_callbacks.hpp	2015-07-22 08:35:07.764263463 +0200
@@ -128,7 +128,7 @@
             }
             case object:
             default:
-                BOOST_ASSERT(false); // must start with string, i.e. call new_value
+                std::abort();
             case key: {
                 l.t->push_back(std::make_pair(key_buffer, Ptree()));
                 l.k = object;