Update matrix.cpp

Fix race condition in getDefaultAllocator and setDefaultAllocator interaction / not threadsafe currently

modules/core/src/matrix.cpp

@@ -222,16 +222,20 @@ public:
 };
 namespace
 {
-    MatAllocator* g_matAllocator = NULL;
+    MatAllocator* volatile g_matAllocator = NULL;
 }
 
 
 MatAllocator* Mat::getDefaultAllocator()
 {
+    if (g_matAllocator == NULL)
+    {
+        cv::AutoLock lock(cv::getInitializationMutex());
         if (g_matAllocator == NULL)
         {
             g_matAllocator = getStdAllocator();
         }
+    }
     return g_matAllocator;
 }
 void Mat::setDefaultAllocator(MatAllocator* allocator)