[#404] Reading past valid address with multisink logger

- Initialising atomic value

See examples at http://stackoverflow.com/q/36320008/706456
This issue was discovered with dr memory tool on Windows platform, Visual Studio 2015 C++ 11

include/spdlog/logger.h

@@ -87,7 +87,7 @@ protected:
     spdlog::level_t _level;
     spdlog::level_t _flush_level;
     log_err_handler _err_handler;
-    std::atomic<time_t> _last_err_time;
+    std::atomic<time_t> _last_err_time {0}; // init atomic value to avoid reads of uninitialised memory
 };
 }