Steps to repro:

1. Download the repro project: https://drive.google.com/open?id=157bdSI17Wvl53vaTGf_cIbLNwT0yFKQi
2. Open 't1BugTerrainScratch' scene;
3. In the Lighting window setting click 'Generate Lighting' button.

Expected result:
Baking finishes successfully. Fallback to CPU PLM happens gracefully.

Actual result:
Editor crashes in PVRJobCompositeLightmap after falling back to CPU PLM.
Some valuable data from the Editor.log:

'Baked lightmap filtering failed during bake. Try disabling the Optix denoiser or lower the Lightmap Size.

(Filename: C:\buildslave\unity\build\Editor/Src/GI/Progressive/Jobs/PVRJobCompositeLightmap.cpp Line: 1846)'
See attached Editor.log file.

Notes:
- Reproducible only in lighting/radeonrays/terrain-alloc branch (this crash was masked under a different RadeonRays crash); will be in trunk when this branch hits trunk;
- Reproducible on Windows 10 with NVIDIA GTX 1070 GPU (8Gb of VRAM);
- The crash is related to Optix denoiser failing to allocate VRAM for denosing the lightmap
- Doesn't reproduce when Filtering is set to None in the Lighting settings;
- Doesn't reproduce with the lightmap size = 1024; Reproducible only when the lightmap size is 4096.