fixed assert in http_connection

src/http_connection.cpp

@@ -493,8 +493,8 @@ void http_connection::callback(error_code const& e, char const* data, int size)
 				close();
 				return;
 			}
-			data = &buf[0];
 			size = int(buf.size());
+			data = size == 0 ? 0 : &buf[0];
 		}
 	}
 	m_called = true;