ImVector: fix undefined behaviour during copy operator if source vector is null. (#5608)

imgui.h

@@ -1765,7 +1765,7 @@ struct ImVector
     // Constructors, destructor
     inline ImVector()                                       { Size = Capacity = 0; Data = NULL; }
     inline ImVector(const ImVector<T>& src)                 { Size = Capacity = 0; Data = NULL; operator=(src); }
-    inline ImVector<T>& operator=(const ImVector<T>& src)   { clear(); resize(src.Size); memcpy(Data, src.Data, (size_t)Size * sizeof(T)); return *this; }
+    inline ImVector<T>& operator=(const ImVector<T>& src)   { clear(); resize(src.Size); if (src.Data) memcpy(Data, src.Data, (size_t)Size * sizeof(T)); return *this; }
     inline ~ImVector()                                      { if (Data) IM_FREE(Data); } // Important: does not destruct anything
 
     inline void         clear()                             { if (Data) { Size = Capacity = 0; IM_FREE(Data); Data = NULL; } }  // Important: does not destruct anything