ImVector: reserve() tweak to avoid undefined behavior warning (#731)

imgui.h

@@ -885,6 +885,7 @@ public:
     {
         if (new_capacity <= Capacity) return;
         T* new_data = (value_type*)ImGui::MemAlloc((size_t)new_capacity * sizeof(value_type));
+        if (Data)
             memcpy(new_data, Data, (size_t)Size * sizeof(value_type));
         ImGui::MemFree(Data);
         Data = new_data;