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/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());
     }
 }