Fixing of AutoBuffer::allocate(nsz) method

AutoBuffer::allocate(nsz) didn't work properly when
(sz < nsz < fixed_size). In this case sz remained unchanged.

modules/core/include/opencv2/core/utility.hpp

@@ -817,10 +817,10 @@ AutoBuffer<_Tp, fixed_size>::allocate(size_t _size)
         return;
     }
     deallocate();
+    sz = _size;
     if(_size > fixed_size)
     {
         ptr = new _Tp[_size];
-        sz = _size;
     }
 }