fix TLS implementation (closes #5967)

modules/core/src/system.cpp

@@ -1084,6 +1084,8 @@ public:
         CV_Assert(tlsSlots.size() > slotIdx);
 
         for(size_t i = 0; i < threads.size(); i++)
+        {
+            if(threads[i])
             {
                 std::vector<void*>& thread_slots = threads[i]->slots;
                 if (thread_slots.size() > slotIdx && thread_slots[slotIdx])
@@ -1092,6 +1094,7 @@ public:
                     threads[i]->slots[slotIdx] = 0;
                 }
             }
+        }
 
         tlsSlots[slotIdx] = 0;
     }
@@ -1115,12 +1118,15 @@ public:
         CV_Assert(tlsSlots.size() > slotIdx);
 
         for(size_t i = 0; i < threads.size(); i++)
+        {
+            if(threads[i])
             {
                 std::vector<void*>& thread_slots = threads[i]->slots;
                 if (thread_slots.size() > slotIdx && thread_slots[slotIdx])
                     dataVec.push_back(thread_slots[slotIdx]);
             }
         }
+    }
 
     // Set data to storage index
     void setData(size_t slotIdx, void* pData)