environment::native_handle fix

closes #332

include/boost/process/detail/posix/environment.hpp

@@ -210,7 +210,7 @@ public:
 
     Char ** _env_impl = &*_env_arr.data();
 
-    native_handle_type native_handle() const {return &_data.front();}
+    native_handle_type native_handle() const {return _env_impl;}
 };
 
 

include/boost/process/environment.hpp

@@ -263,7 +263,7 @@ public:
         auto st1 = key + ::boost::process::detail::equal_sign<Char>();
         while (*p != nullptr)
         {
-            const std::size_t len = std::char_traits<Char>::length(*p);
+            const std::ptrdiff_t len = std::char_traits<Char>::length(*p);
             if ((std::distance(st1.begin(), st1.end()) < len)
                  && std::equal(st1.begin(), st1.end(), *p))
                 break;

test/environment.cpp

@@ -30,6 +30,7 @@ BOOST_AUTO_TEST_CASE(empty,  *boost::unit_test::timeout(5))
     BOOST_CHECK(ev.empty());
     BOOST_CHECK_EQUAL(ev.size(), 0u);
     BOOST_CHECK_EQUAL(ev.end() - ev.begin(), 0);
+    auto nh = ev.native_handle();
     ev["Thingy"] = "My value";
 
     BOOST_CHECK(!ev.empty());