Coverity: Fix 1595225 Data race condition

Author: kbenzie

source/loader/layers/validation/ur_leak_check.hpp

@@ -136,6 +136,7 @@ struct RefCountContext {
     void clear() { counts.clear(); }
 
     template <typename T> bool isReferenceValid(T handle) {
+        std::unique_lock<std::mutex> lock(mutex);
         auto it = counts.find(static_cast<void *>(handle));
         if (it == counts.end() || it->second.refCount < 1) {
             return false;