Merge pull request #995 from Unity-Technologies/decal-system-fix2

NPE fix

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Decal/DecalSystem.cs

 
         public void Cleanup()
         {
-            m_DecalAtlas.Release();
+            if (m_DecalAtlas != null)
+                m_DecalAtlas.Release();
             CoreUtils.Destroy(m_DecalMesh);
         }
     }