[#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/details/mpmc_bounded_q.h

@@ -159,9 +159,9 @@ private:
     cell_t* const           buffer_;
     size_t const            buffer_mask_;
     cacheline_pad_t         pad1_;
-    std::atomic<size_t>     enqueue_pos_;
+    std::atomic<size_t>     enqueue_pos_ {0};
     cacheline_pad_t         pad2_;
-    std::atomic<size_t>     dequeue_pos_;
+    std::atomic<size_t>     dequeue_pos_ {0};
     cacheline_pad_t         pad3_;
 
     mpmc_bounded_queue(mpmc_bounded_queue const&) = delete;