Fix #5196: setLowLODElement crash when LOD element is a building

[x6c85]

Summary

setLowLODElement could crash the client when both the high-detail element and the LOD were buildings. After #5139 a building in another dimension (or one that failed to allocate) has no native GTA entity (m_pBuilding == nullptr), but CClientBuilding::SetLowLodBuilding still called SetLod on that pointer.

The MTA-side high/low link is now stored even if the native entities are missing, SetLod is only called when both exist, and Create() restores the GTA LOD link once the building is actually created.

Motivation

Fixes #5196. Building-to-building LOD is supported (wiki: createBuilding + createBuilding + setLowLODElement); it just shouldn't crash. This matches the report that it reproduced in a gamemode but not always via runcode.

Test plan

local b = createBuilding(5427, 2170.9765625, -1461.125, 25.0859375, 0, 0, 0, 0)
local bLOD = createBuilding(5581, 2170.9765625, -1461.125, 25.0859375, 0, 0, 0, 0)
setLowLODElement(b, bLOD)

Client should not crash. Also check:

• same snippet while the local player is in a non-zero dimension
• createBuilding + createObject(..., true) + setLowLODElement still works
• changing the player's dimension and coming back still shows the LOD pair

Checklist

• Your code should follow the coding guidelines.
• Smaller pull requests are easier to review. If your pull request is beefy, your pull request should be reviewable commit-by-commit.

[FileEX]

Please resolve conflicts

[FileEX]

Please fix the clang-format issues