Fix potential race condidion when in logger copy ctor

Don't copy other.trace - just create a new one with same size instead

include/spdlog/details/backtracer.h

@@ -17,14 +17,19 @@ namespace details {
 class backtracer
 {
     std::mutex mutex_;    
+    size_t n_messages_;
     circular_q<log_msg_buffer> messages_;
 
 public:
-    explicit backtracer(size_t n_messages) : messages_{n_messages}
+    explicit backtracer(size_t n_messages) : n_messages_{n_messages}, messages_{n_messages}
     {}
 
-    backtracer(const backtracer &other) : messages_{other.messages_}
-    {}
+
+    size_t n_messages() const
+    {
+        return n_messages_;
+    }
+
     
     void add(const log_msg &msg)
     {

include/spdlog/logger-inl.h

@@ -25,7 +25,7 @@ SPDLOG_INLINE logger::logger(const logger &other)
 {
     if (other.tracer_)
     {
-        tracer_ = std::make_shared<details::backtracer>(*other.tracer_);
+        tracer_ = std::make_shared<details::backtracer>(other.tracer_->n_messages());
     }
 }