Wrong iterator for FRuntimeMeshLOD::Sections TMap cause crash for sectionIds of non sequential or large values

[Alex-Folts]

So here: https://github.com/KoderzUnreal/RuntimeMeshComponent/blob/1c8a9d261c6d074e2731ec8474d6ec05d3356ecb/Source/RuntimeMeshComponent/Private/RuntimeMesh.cpp#L1152 you iterating through TMap using for loop, the problem is if section have 1 element and its id != 0, it will try to acess TMap element with key = 0(look for loop above), and will crash. I.e. if you create sections out of order or just create it with some random Id - you will got crash. Assertion failed: Pair != nullptr [File:C:\Program Files\Epic Games\UE_4.23\Engine\Source\Runtime\Core\Public\Containers/Map.h] [Line: 508]

This is one of proper ways of iterating through TMap: https://answers.unrealengine.com/questions/814439/how-to-iterate-through-a-tmap-via-range-based-for.html

To reproduse bug, just call CreateSection with section id of some large number(even 1 will work if its first section). I'm using UE_4.23.

I was changed code of RuntimeMesh.cpp like so and it seems work now:

                        //for (int32 SectionId = 0; SectionId < LOD.Sections.Num(); SectionId++)
            //{
            //  RenderProxy->CreateOrUpdateSection_GameThread(LODIndex, SectionId, LOD.Sections[SectionId], true);
            //  bHadAnyInitialized = true;

            //}
            for (const TPair<int32, FRuntimeMeshSectionProperties>& pair : LOD.Sections) {
                RenderProxy->CreateOrUpdateSection_GameThread(LODIndex, pair.Key, pair.Value, true);
                bHadAnyInitialized = true;
            }```