Fixed assert triggering when a window has zero rendering but has a callback (#810)

imgui.cpp

@@ -2541,8 +2541,8 @@ static void AddDrawListToRenderList(ImVector<ImDrawList*>& out_render_list, ImDr
     }
 
     // Draw list sanity check. Detect mismatch between PrimReserve() calls and incrementing _VtxCurrentIdx, _VtxWritePtr etc.
-    IM_ASSERT(draw_list->_VtxWritePtr == draw_list->VtxBuffer.Data + draw_list->VtxBuffer.Size);
-    IM_ASSERT(draw_list->_IdxWritePtr == draw_list->IdxBuffer.Data + draw_list->IdxBuffer.Size);
+    IM_ASSERT(draw_list->VtxBuffer.Size == 0 || draw_list->_VtxWritePtr == draw_list->VtxBuffer.Data + draw_list->VtxBuffer.Size);
+    IM_ASSERT(draw_list->IdxBuffer.Size == 0 || draw_list->_IdxWritePtr == draw_list->IdxBuffer.Data + draw_list->IdxBuffer.Size);
     IM_ASSERT((int)draw_list->_VtxCurrentIdx == draw_list->VtxBuffer.Size);
 
     // Check that draw_list doesn't use more vertices than indexable (default ImDrawIdx = 2 bytes = 64K vertices)