ImVector: skip memcpy in operator= if Data isn't initialized to play nice with -fsanitize=undefined. (#8874)

docs/CHANGELOG.txt

@@ -89,6 +89,8 @@ Other Changes:
 - Misc: fixed building with IMGUI_DISABLE_STB_TRUETYPE_IMPLEMENTATION. (#8794)
 - Misc: removed more redundant inline static linkage from imgui_internal.h to 
   facilitate using in C++ modules. (#8813, #8682, #8358) [@stripe2933]
+- Misc: ImVector: skip memcpy in operator= if `Data` isn't initialized in order
+  to play nice with -fsanitize=undefined. (#8874) [@i25e]
 - CI: Added SDL3 builds to MacOS and Windows. (#8819, #8778) [@scribam]
 - CI: Updated Windows CI to use a more recent SDL2. (#8819, #8778) [@scribam]
 - Examples: SDL3+Metal: added SDL3+Metal example. (#8827, #8825) [@shi-yan]

imgui.h

@@ -2181,7 +2181,7 @@ 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); if (src.Data) memcpy(Data, src.Data, (size_t)Size * sizeof(T)); return *this; }
+    inline ImVector<T>& operator=(const ImVector<T>& src)   { clear(); resize(src.Size); if (Data && 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